KAP 5.1, Page 26
"...Subtract your age from the current game year to find the year your character was born, and record the answer here.
As a default, a new campaign begins in 485 A.D., and characters starting at age 21 were born in 465."

485-21 would be 464.

"Book of Knights & Ladies" and the "Pendragon Gamemaster Characters" both suggest that it is the example that is wrong, rather than the text. Also the preconstructed characters on p. 35 are age 21, born in 464.

So this could be corrected by changing either the starting age, or the birth year.

As noted in the errata thread on rules forum. If the birth year was adjusted then an entry in the Salisbury history would also need to be adjusted.

p. 48, A note in the entry for 465
"In this year your character is born, assuming the default starting age of 21."

I started to make a list of every time the book specifically refers to the starting age of 21, but there are a bunch.

Actually, given the way Pendragon Aging & Childbirths works (in Winter Phase), the birth year should be 463. Here is why:

A) If you follow KAP 5.1 introductory scenario, it is pretty obvious that it is supposed to happen in the Autumn (usual hunting season and it is followed straight away by the Winter Phase) and the adventure should be set in 484, since according to GPC, they are knights in 485 (actually, KAP 5.1 intro scenario and Family History don't mesh perfectly, more of that later). To be accurate, the PKs should be 20 at the time of the adventure (still squires), and they should age TO 21 during their first Winter Phase but skip the Yearly Training (Step 7), since they are already made as 21-year old knights, not 20-year old squires. Alternatively, you could make this even more exact by giving them only 3 miscellaneous choices in character generation instead of 4, which would reflect one year less training. And then it would work beautifully.

B) In any case, since the Aging and Childbirth both are recorded in Winter Phase, and the PKs are 21 years old AT THE START of 485, that means their previous 'birthday' was in Winter 484. Since that was their 21th birthday, they must have been born in Winter 484-21 = Winter 463. So during 463. Needless to say, this makes the problem of marriages and births in the Family History in KAP 5.1 all the greater.

My preference:
1. Have the Father marry a non-heiress in 461 or so, as soon as he is 21. The PK's manor is an ancestral manor from Father's side, not from Mother's, thus allowing patrilineal cousins have a claim if your main line dies out. Also, even after the Night of the Long Knives, heiresses should not be so plentiful that 3-6 almost freshly knighted Father-knights with ~1200 Glory each get one!
2. The PK is born in 463.
Thus, no special rules or explanations are needed, and heiresses stay rare, as they should be (see this thread for more: http://nocturnalmediaforum.com/iecarus/forum/showthread.php?2091-Heiresses-Revised&p=21900&viewfull=1#post21900 ).

However, if you wish to keep the heiress idea (boo!) then here is what you can do:
1. The Father marries the heiress right after the Night of the Long Knives, in late 463 (or in a pinch, at the start of 464).
2. The PK is born in (Winter) 464.
3. Simply knight the 20-year old PK a year in advance, in 484, due to the losses incurred in that year's fighting; as implied in the Family History, Uther is desperate for more knights in 485. You can also handwave the extra 484 winter training (at the end of the intro) and the extra year (the 20-year old using the chargen rules of 21-year old) as these PKs being exceptional for their age (getting 2 years of training & experience for 'free'). Granted, you can also keep the birth year at 465 and simply make the PKs exceptional 19-year olds. But is it worth it? Changing the rules so that ONE easily changed number stays the same? If you don't keep the heiress idea, there is practically no reason not to change the numbers to match what they should be. Or you can take some of those miscellaneous picks away from chargen, to reflect the youth and inexperience of these new knights. Don't worry, they will make up for it.

The Discrepancy between KAP 5.1 Family History and KAP 5.1 Intro & GPC:
In KAP 5.1 Family History, Uther is calling for squires who are ready to be knighted to gather at Windsor Castle in Spring 485. However, GPC (p. 25) is explicit that the intro scenario & the knighting & the winter phase have been completed in the previous year (484) and the PKs are fully fledged knights at the start of 485. There is no knighting ceremony at Windsor in GPC, since why would there be? The PKs are already knights!

Book of Uther starting in 480:
Of course, using BoU, the birth years (Father and PK) would have to be pushed back another 5 years anyway. See this thread: http://nocturnalmediaforum.com/iecarus/forum/showthread.php?2552-Starting-the-Campaign-in-480-rather-than-485-Things-to-consider&p=21899&viewfull=1#post21899 .