Reverse aging
Reverse aging was a medical condition in which a lifeform experienced age-related changes, in whole or in part, in reverse of those typical to its life span.

An individual in temporal flux could simultaneously experience both reverse aging and accelerated aging across different parts of their body. (; )

Those exposed to the antimatter universe age in reverse to those in the prime universe, described by Spock as "If the flow of time is reversed, then one is born at an old age, and dies in infancy. Your descendants are born before you, and your ancestors are born after you." As the crew of the  experienced the reverse aging effects caused by entering the antimatter universe in 2270, they found that returning to their normal universe caused the reverse aging process they were experiencing to stop. ()

The inhabitants of Cerberus II developed a drug and herb combination that could reverse the effects of aging. Starfleet Admiral Mark Jameson used and abused the agent, initially to treat his Iverson's Disease, changing his appearance from that of an 85 year-old man to one much younger. ()

Data and Miles O'Brien were able to use the transporter to reverse and restore Katherine Pulaski to her normal age in 2365. ()

A bite from an alien horse caused a command division ensign to experience reverse aging on one side of his body, and advanced aging on the other. ()

Medical conditions
Physiology