Kind of an old movie. A rich gal living in a mansion, plays piano. One day she runs off into the woods, and sees a horse drinking water. She tries to drink too and a boy stops her. To see if you drink it you will be immortal. Then she finds out there's a secret family living there. They are all immortal. She falls in love with the guy... n on..

Tuck Everlasting (2002) (http://www.imdb.com/title/tt0283084/)
-or-
Tuck Everlasting (1981) (http://www.imdb.com/title/tt0081664/)

Yes! Dats it! Fankuuuuuu :))