Cocoon

Cocoon is a science fiction film directed by Ron Howard in 1985 about a group of elderly people who are rejuvenated by swimming in a pool that is charged with life-force and was intentionally left there for aliens to use.
Approximately 10,000k years ago, a group of aliens from the planet Anterea formed an outpost on Earth at an area known as the legendary and mythical Atlantis. Once Atlantis sank due to an earthquake, 20 alien members remained behind so the rest would have sufficient lifeforce to return to their home planet.

The story goes on with the aliens returning back to the outpost to retrieve the remaining members. They charge up a pool with life-force but unfortunately, elders from a retirement-home sneaked into the pool and unknowingly used the lifeforce. Although, one refused to use the powers and spilled the secret, resulting in all the elders using the pool.

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Source: http://en.wikipedia.org/wiki/Cocoon_(film)