A boy is 17 forever because he drank from a river that keeps you young forever. And he meets a girl who wants to drink from it to stay young also. Its based on a book.

Tuck Everlasting. There's been more than one film made on the story, here's some info on the film from 2002:
Tuck Everlasting (2002) - IMDb (http://www.imdb.com/title/tt0283084/)