Jack (1996)

"He's a healthy ten year old who's growing four times faster than normal. Now he's about to take off on the biggest adventure of his life... 5th grade."

Jack Powell suffers from an affliction that makes him grow four times faster than normal, so the 10 year old boy looks like a 40 year old man. After years of being tutored at home, Jack convinces his overprotective parents to send him to public school. The children don't know what to make of Jack, but with the help of his fifth-grade teacher, he makes an effort to win them over.