Okay, yeah, fine, not that old. I guess it just felt older than it was, because I was looking through my game and thinking, "I remember none of this." That probably says more about my memory than the age of the game, lol.
As for which game it was, I vaguely remember that I was semi-simultaneously playing SoulSilver, Diamond, Ruby, and FireRed in an attempt to get as close to a (legitimate) complete Pokedex as I could. So those Pokemon could have come from any of those games (even though I found them on SoulSilver).
Anyway, to get back to the topic of Animorphs (heh, sorry), yeah, Tobias's aging was never covered in the books, but Blaze brings up a good point about DNA. According to some studies, it's actually the DNA that ages, and as it replicates throughout your lifetime, it shortens, and that's the reason why we have a limited life-span.
But does that mean that morph DNA keeps aging, or does the morphing process preserve it somehow? I think the biggest argument against the idea of morphs aging is the fact that the Animorphs have fly morphs. If those morphs aged normally, they'd be dead in a matter of days, since that's all the longer flies can live.
Of course, as RYTX pointed out, all that has been discussed before, so I think I might be off-topic again by bringing that up too.