I get that part of the game is the retiring of knights when they get too old or become barons and the like but I would be interested to hear if anyone has kept playing their old or ennobled knight and how it went. I guess if you had been made a Duke or the like you would get so much yearly glory you could hold off the ageing penalties virtually indefinitely. I suspect it would be a bad idea but it seems plenty of the knights of the round table are higher nobility and still undertake knightly adventures.

On a related note do you just roll on the ageing table once per year when over 35 or do you roll multiple times at 45, 55 etc?

You might be interested in looking over this thread:
http://nocturnal-media.com/forum/index.php?topic=2080.0
where I talk about the aging probabilities and the like.

I do not think that there needs to be additional aging rolls at 55 and 65, since what tends to happen is that your stats get low enough already that you are practically useless, and will die in one blow if you ever go off to serious fighting already.

As a rough rule of thumb:
35 - 44: You start to decline, but unless you get unfortunate with your rolling, you can probably keep up. Especially if you are racking a lot of Glory points to forestall the worst hits from aging.
45 - 54: The decline is noticeable. You can still fight if you need be, but your stats are clearly sub-par, as is your damage.
55 - 64: The decline continues. While you are still likely to be alive by the end of this decade, your stats are so low that you will certainly not belong in a battlefield anymore.
65 - 74: Most knights die of old age.
75+: Only very rare or very Glorious knights survive this long. (Arthur, famous RTKs benefit from high glory and high starting stats, as well as GM save when it comes to aging rolls.)

I have not had any PKs in my campaign reach this far. The oldest made it to 37, I think, before dying at the Battle of Badon. Another PK died around the same age, having become an RTK, and then perishing against a big giant on a quest. So it hasn't come up so much. We have two player chars at 36 and 35 now again, so lets see how they do. The older is an RTK, the younger a lady.

Given that our PKs seem to be gaining around 500+ Glory per year, that would normally not be enough to keep up with the stat loss (roughly 1 / year), unless they have a more exciting year and gain a full 1000.

As far as I recall, the landholding Glory is capped at 100, so even Earls and Dukes would only gain an additional 100 from that. They could, of course, sponsor feasts and tournaments and gain extra glory from those.