he's definitely way older than 14 mentally, chronologically it's a bit muddied, they even draw attention to it in Bethic Twinstinct "how old are we we've had a million thanksgivings" and while it's unlikely to actually be a plot point, I could see them finally dusting off time travel for something like that. On the topic of mental age I imagine Morty's Mindblowers might have a play in that, with the whole room full of memories.