I just feel that having Billy grow up looses the whole hook of the character.
He's a kid (10, 12, 14, doesn't matter) that turns into an adult superhero. An 18 year old turning into an slightly different looking adult just doesn't have the same magic.
If you want to do some kind of future story or elseworlds to see where he goes, that's fine, but there's no point in taking away the basis for the actual character.