The GPC lists 501 as Tristram's birth year, but then refers to him as a child in 522. College jokes aside, considering that the Isold events occur in his 30's this way, should his birth year been 510?

I just ran across this issue myself - like you say, given that Tristram only enters the world of chivalric adventure after his debut against Sir Marhaus in 529 I think it makes sense to move his birth back a decade or so, putting him properly into the generation of courtly/questing knights who come in to their fame after the wars of the early reign.

IMG he'll be getting born in 511 - I have already introduced his father as a sympathetic background NPC in my game, so my PKs might even get to interact with him as a youth.

Regards
Luke