Ok, in all honesty, I think that he probably did
not conciously choose to age, I just don't think there was any other way for him to adapt to the idea of being dead. Afterall, Seymour ages, as far as I know, because, if I'm right about this, Seymour died when
(SPOILER)he was on his way to defeat Sin, and that was when he was just a kid . However, I'm not 100% sure on anything that happened to Seymour, because
(SPOILER)I still haven't gone inside Sin or beaten the game , so don't ruin things for me :D?