Things that keep you young forever:
The Fountain of Youth
The juice from Death Becomes Her
Being Peter Pan
Things that keep you from dying (but don't necessarily keep you young):
The Philosopher's Stone*
Being dipped in the river Styx
Things that keep you at the same age forever (whether young or old):
Drinking the spring water from Tuck Everlasting
Being revivified by The Pie Maker
Being bitten by a vampire
Cursed Aztec Gold in Pirates of the Caribbean
*Depending on the incarnation, this could also belong in the first category.
What am I missing?