I have a character concept in mind for a backup character I am making which involves a character being both very old and very young at the same time. So the idea is this character (as a kid) pickpockets a very powerful wizard and when the wizard finds out he curses the character to never be able to age. Which at first sounds great but he is now forever trapped in the body of a kid. It takes the character a few years to figure out he is cursed and when he does he starts looking for a way to remove the curse. The catch is that by the time he is able to remove the curse he has become so old that without its magic he would die. So now he is looking for true immortality so that he can remove the curse and grow up without dying.
This type of character concept is nothing new but my question is how would this type of character be represented mechanically? There are rules for young characters, and there are rules for old characters but what about both at the same time. I know most of this will fall into ask your dm/gm territory but I am asking, if you where the dm/gm what would you do?
Thanks for any help.