Merlin Sickness

Merlin's Sickness is a condition in which a character ages backwards, going from an elderly or mature state to a state of infancy and eventually dissolution. This is not the same as the character actually going backwards in time while aging backwards.

Empath: The Luckiest Smurf stories
This condition is listed as just Reverse Aging in Papa Smurf's books of unusual medical conditions in the EMPATH: The Luckiest Smurf story series.