Ben 10

S04:E03 - Don't Drink the Water

In Florida, Ben and Max accidentally come in contact with water from The Fountain of Youth. As a result, Max transforms into a kid and Ben into a toddler, with Gwen having to deal with both of them. The trio then must track down Hex, who's found the source of the fountain, and prevent him from drinking the water that will make him immortal -- and unstoppable!