I am trying to find a fantasy movie I seen as a child, late 60s or early 70s.
They climb a mountain and enter into a cave, then they come out the other side of the cave and enter a vally where no one grows old and there is no sickness and they are told they can't leave but they do and when they go back through the cave they instantly age.

Tarzan's Magic Fountain (http://www.tcm.com/tcmdb/title/92398/Tarzan-s-Magic-Fountain/full-synopsis.html)?

Legend of the seeker is a TV series but sounds just like one of the episodes

That sort of reminds me of Lost Horizon.

The 1973 version (a musical): Lost Horizon (1973) - IMDb (http://www.imdb.com/title/tt0070337/)

The older version from 1937:Lost Horizon (1937) - IMDb (http://www.imdb.com/title/tt0029162/)