Substitution
Deacon Blues
I don't know, I think they're in on it. I mean, if that's the case, then how come they look equally as young? I'll tell you why.Or he drains the youth of his partners to prolong his life. There's also that theory.
It's because they're stealing the youth from the Pokemon. Why else would Ash coincidentally leave all his Pokemon barring Pikachu with Professor Oak? Because he's drained them dry, and has no reason to keep them around.