The Fountain of Youth is a Limited Edition Idol which when touched with the hand icon, does what it suggests- it reverts your pets back to their young state.
Although, there's a catch- It's only temporary. After a little while, your pet will grow back to it's original state. This Idol is permanent, however, so you should be able to use it whenever you feel necessary.
The Fountain of Youth was available for 88 Facebook Credits, or just under $9.