We are reaching a point where some player characters are dying before their children are coming of age, leaving older knights to act as regents/stewards until the rightful heirs are of knighting age. How do you handle this for knights who should be in their 30s as simply allowing the aging rolls/extra squire-hood rolls seems to be generating knights who are far superior to anyone else, kind of nullifying the loss of a character with years of 'player adventuring time'.

We are reaching a point where some player characters are dying before their children are coming of age, leaving older knights to act as regents/stewards until the rightful heirs are of knighting age. How do you handle this for knights who should be in their 30s as simply allowing the aging rolls/extra squire-hood rolls seems to be generating knights who are far superior to anyone else, kind of nullifying the loss of a character with years of 'player adventuring time'.

EDIT:
Wow, I got a bit wordy. Let me just summarize it quickly here: We have not found older knights to be a problem in our campaigns, so I don't think you need to be that worried about it. :) Below are some reasons why.

I originally had this as my second point, but I think it deserves to be the first point I make:
A character's death is not supposed to be a PENALTY in Pendragon. It is simply something that happens in a generational game. All men must die, but it is the FAMILY that goes on. If your character has a 30-year old brother to look after his 12-year old son, this is a GOOD thing. It keeps you (as the player) involved in the Family's fate, allowing you to contribute to the Family's continuing story and look after your future PK. What of it that you have a 30-year old character? In Pendragon, it is fully possible to have a 55-year old veteran Round Table Knight adventure alongside of a 21-year old freshly knighted kid. (Granted, I would probably suggest 'retiring' the RTK at that point, simply due to the disparity in status and skills. But I might not, depends if the character chemistry works; the RTK could easily become more of a mentor character, grooming the new generation of heroes. This can be an interesting group dynamic, too, rather than everyone being equal.)

Then to the rest:
Far superior to whom? To 21-year old starting knights? Yes, for sure. That is to be expected if they have been doing this knight thing for 10-15 more years than the beginner knight.

However, they are NOT as good as a knight who has played his way through those 10-15 years. Those older knights get only the Yearly Glory, so they are likely well behind in total Glory. Furthermore, they don't get experience checks until they actually enter play, only the Yearly Training. This means that they will miss out on a numerous skill checks, which, on average (and in our experience), means that they need to spend about 2 years' worth of Yearly Training to each Sword, Lance and Horsemanship to match just the experience checks that the other PKs got during play. And this is totally ignoring the fact that during those 10-15 years, the PKs have been making friends (and enemies, true), gotten the trust of their liege (hopefully) and with the accumulated good will and Glory total are poised to be rewarded with offices and other goodies. Whereas the new old knight is just an average knight of his age, not remarkable in any way. The Son of the deceased PK inherits the PK, and likely gets at least a fraction of that good will and family alliances as well, which the old knight might not get (or he might, depending how close he was to the deceased PK, a brother probably would qualify to some, a cousin likely not significantly anymore). Sure, if you start the play with a 34-year old knight, you will get extra 13 yearly trainings, but you will start rolling for aging at the next Winter Phase. Aging chips away about 1 point per year, on average, and you cannot increase your stats anymore. So unlike the young PKs, you start declining, while they are still improving.

Furthermore, Pendragon system is remarkably forgiving of Age disparity. It is not like in D&D, where the difference between a 1st level mage and a 20th level mage is like between a peasant (and a sickly one at that) and a demi-god. Here, let me show you an example:

21-year old combat maximized starting knight:
SIZ 18
DEX 10
STR 15
CON 14
APP 10
= 67 or to put it in another way, 60+3 CON+4 picks to stats
Skills: Sword 15, Lance 15, Horse 15 (Sword to 15, 10 points split between Lance and Horse)
Damage: 6d6
HP: 32

34-year old combat stat maximized knight (13 yearly training picks to stats):
SIZ 18
DEX 13
STR 18 (while this doesn't add more Damage, it does give the old knight a bit of a buffer before Aging causes his Damage to drop)
CON 21
APP 10
= 80
Skills: Sword 15, Lance 15, Horse 15 (Sword to 15, 10 points split between Lance and Horse)
Damage: 6d6
HP: 39

So what is the difference? 7 hit points (which is not much, maybe one more good hit, or two small ones), a higher Major Wound Threshold (useful, but a critical probably still takes him out, and non-criticals are unlikely to cause a major wound on the 21-year old, either), and higher DEX (quite useful, not falling as easily, but SIZ 18 means that Knockdown is not triggered that often to begin with against 5d6 enemies, so it shows up less often than you might think). Still, he is NOT in a class of his own. The damage and the skills are still the same. They both hit as hard and as often. You can put these guys against same kinds of enemies, and they will do about just as well.

34-year old combat skill maximized knight (13 yearly training picks to skills):
SIZ 18
DEX 10
STR 15
CON 14
APP 10
= 67
Skills: Sword 20, Lance 20, Horse 18 (Sword to 15, 10 points split between Lance and Horse)
Damage: 6d6
HP: 32

I'd argue that this guy is much better fighter than the stat optimized one: the higher skills allow him to always get the shield, and likely to hit the enemy and hence not himself get hit as often. Still, that being said, the combat between skill 15 and skill 20 is not a foregone conclusion. Skill 20 is likelier to win it, but the chance to critical is the same, still, and those are the fight enders. I think I was crunching the numbers for this way back when and came up with something like 3:2 odds for the Skill 20 landing a hit vs. Skill 15 landing a hit. Again, Skill 20 has an edge, but he isn't a demigod by any stretch of imagination.

Also, outside of combat, both of the above old knights are as unskilled as the starting knights, meaning they are equal, unable to hog the spotlight there. And as seen from the above, they are not Combat Gods, either. As soon as those sweet sweet experience checks start rolling in, the young knights get better, and the Aging starts to take its toll on the old knights.

How about a skill focused old knight? Well, if he focuses on lower skills (<15), he gets 1d6+1 skill points per year: average 4.5*13 = 58.5 = 59. Most of the skills start from 2 or 3, meaning he will have to spend about 25 points to get two of them to 15. Or he might focus on skills that he already had at 10 from Chargen. This means that he probably would have a line-up like this:
3 starting non-combat skills at 10+5=15 (15 points)
Battle at 10+5 (5 points)
3 more skills at 15 (about 38 points, with a point or two left over, maybe use those for First Aid?)
= 7 additional skills at 15.
So yeah, he is pretty good. But is he really that dominant? Since you are comparing him to other knights, I am assuming that there is a playgroup with at least two other knights, maybe more, each of whom has 3 non-combat skills at 10 (or better, especially given the Family Characteristic). Assuming that the GM has recommended them not to have all the same skills, and the player of the old knight is not being a dick, there should still be skills where the younger knights are the best in the group. And in many skills, like performance skills (dancing, playing, singing), it doesn't usually matter if someone is better, since you are rolling to see if you succeed, regardless of their roll. Even in cases like Courtesy and Orate, you might have a chance for rolls from other PKs, not just the 'designated spokesman'. And skill 15 can still fail, 25% of the time. In short, what the old knight would contribute is the breadth of experience in this case, making the group able to tackle various challenges, without stealing the spotlight from the other, younger, PKs.


Finally:
It is very unlikely that you have PKs that are all exactly the same age. Thus, you have some natural variance there as well, further muddling the perceived advantage that the 34-year old knight would have. Assuming he even is 34-year old; he could easily be younger (or even older). And it is unlikely that he is fully focused on any of the three builds above (stats, combat skills, other skills): more likely is that he is spreading things around, which means that he will have slightly better stats (STR 18 as buffer, CON 16 = 5 years), combat skills (like 17 instead of 15 = 4 years, assuming Horsemanship stays 15), and non-combat skills (4 years = 18 points => three skills from 10 to 15, and one to 13). And this kind of a character, while slightly better, is not going to outshine everyone.

EDIT:
Some personal experiences:
We finished one GPC campaign, where towards the end, we tended to have one or two Round Table Knights (RTKs) in their 30s, adventuring alongside with young knights in their 20s, who might have been their squires just a year or two previously. It still worked, and as those RTKs got killed fighting Dragons and whatnot, those younger knights found that they were now the oldest knights in the group, mentoring the young knights who joined the group to take their fathers' place (often being the sons of the older knights). It all comes around, is what I am trying to say, and also, the gap (in skills, and definitely in Glory) between those RTKs and the new knights was much much wider than it would have been between a 34-year old knight out of character generation vs. a 21-year old knight out of character generation.

In our current GPC campaign (slightly different group), the ages of the knights spanned from 18 years (we have houserules for younger characters than 21 being knighted) to 35 years. Despite that age disparity, arguably the 18-year old was the deadliest on the battlefield (granted, an inherited magic sword tends to help, along with high passions). Point is, the 35 year old, despite having been played through to that age, didn't dominate the game at all.

Another thing I didn't point out was that if you are using the 'recommended' pacing, 1 session per game year, those old knights will decline quickly and the young knights will blossom fast. We don't, by the way; we average about 4 sessions per game year: Easter Court (planning for the next year, politicking), Summer Adventure (usually 2 sessions), Autumn Wrap-up (family issues, Winter Phase). And still the age disparity has not been a problem.

The one thing I will add to Morien's treatise is agreement with the statement of "It is the family that is important." Too many times have I seen players invest hours in creating their character, then outlining their family, their manor (if they have one), the play time of getting a wife and a son and throw it all away because the character is killed off while the heir is still very young. The player creates a new character and when the original heir is old enough to play, the player really doesn't care because he has invested those same hours (and now possibly years) with that new character. Having a brother, uncle, cousin, sister, etc. who can keep that line going in the story really helps in that case.
There are lots of potential scenarios involved in this as well. A male relative who has a son of his own, but not heir to this fine manor he is now the guardian of... A wife who will remarry and lots of potential stories along with that idea, and so on.

Too many times have I seen players invest hours in creating their character, then outlining their family, their manor (if they have one), the play time of getting a wife and a son and throw it all away because the character is killed off while the heir is still very young.

Alas, I have seen this too. This is why I am strongly in favor of giving the PKs a strong family base to build on, from the get-go, with a couple of brothers as spares, who can step in and be the caretakers. And trying to get the starting PKs married straight away, so that they can start getting those heirs of their own.

Good point about the family drama, too. I guess you can always do that drama with the widow's new husband, too, but alas, the new husband doesn't have anything to do with the original family, but is part of a new one. Hence, you don't quite get the same continuum, although it is MUCH better than starting a 100% new family somewhere on the other side of the county.

In our new playthrough of GPC, we have had three cases where the only surviving heir was an underaged female:
1) The player switched to the uncle (remaining one of three), overseeing the manor. However, since this third character had a slew of sons, it was decided (by the GM in consultation with the player) that it would be best for story purposes to have the heiress die of an illness and thus let the manor fall to this thriving branch. Rather than have her marry off with the manor.
2) The player switched to the uncle (remaining one of three), overseeing all the manors of the family (his eldest brother had sons, and the middle brother, having earned a manor through his own actions during the Anarchy, had the daughter). This uncle character was a bit unlucky in love (his wife got lured away by a Faerie Prince, never to return), so he has decided to remain unmarried and leave his nephew to inherit his manor (again, earned in play). The heiress is possibly going to become a quite interesting lady character, since a (faerie?) lion owed a boon to her father. Once she grows up, the lion will appoint itself as her bodyguard. (Think of Yvaine, the Lion Knight.)
3) In the final case, the player switched out to another family, although possibly returning to the original one, depending how things go with the new one.