[ Fountain of Youth? Naruto has never heard of anything like that, so he can't really grasp the gravity of the situation or the weight that it holds. All that comes to mind is.. the word Youth. ]

But you're old.

[ He squints, as if trying to work out if Ban is messing with him again. Ban is, by no stretch, old enough to be called old, but he's older than Naruto and therefore.. You get the idea. He can't understand how someone as not-a-teenager as Ban could have drank anything to do with youth. ]

Shouldn't it make ya young if it's called the Fountain of Youth?? If it keeps you at the age you are, then it's just an live-forever fountain or whatever.