honestly considering the average life expectancy of ninja/anbu in particular he might be middle aged

i think that, going by his actual age (26) he's actually super mid-twenties for shinobi culture in general. most characters seem to get treated as the actual age they are when they're in the village, and then treated like weapons any other time. such is the way of things. THAT SAID. ANBU? we don't see much of anbu culture, and because of that i think i agree with you. i think by your mid-late twenties you either die in anbu or you are gently removed from anbu before you become a danger to yourself or the people around you. or maybe people just leave to do normal ninja work. start craving a social life. i don't think many people stay in anbu till their 30s, if only because anbu seems pretty cut off from the rest of konoha and if you want more ninja war orphans running around eventually, people generally have to be allowed to settle down and have kids about it, which seems unlikely for most anbu black ops folks.

i think new anbu recruits (i imagine they're largely late teenagers) look at Yamato and go "he oughta be leaving us soon. one way or another."