The "change in age" in FFI is kind of... umm... unverifiable. That only happens upon class change in the NES and MSX versions, and it's only perceptual, no one actually states that the characters have aged any. It also happens because of "Bahamut's gift", which is instantaneous. In later versions of the game (WSC, PSX, GBA, FoMA) the sprites remain SD style after the class change. The other big factor is that it's optional. If you don't do it, the game remains a tiny bit harder and you have to level up more. This means your adventure takes longer, but you would appear to remain younger. See what I'm saying?