So, of the Astros developed players on the team currently we have Buchholz (24), Nieve (turns 24 tomorrow), Lidge (26 at first callup, injury plagued), Oswalt (23 when he came up), Qualls (26 when he came up), Wandy (26 when he came up, but how old was he as Eny Cabreja? - hardly a good example), Berkman (23), Biggio (23), Burke (ripe old age of 24), Taveras (22), Ensberg (26), and Adam Everett (25).
(I've tried to make sure that I didn't start them when they got a september call-up but a mistake or two is possible).
For those scoring at home, that's 7 in the 24 and younger crowd, and 5 in the 25 and up crowd. That also includes injury plagued lidge and the incredible aging Eny. I guess I define the word exception differently than you do.