Life clock:
The user can both see and manipulate the number of years of life of any living being including their own. The numbers appear in the form of a digital countdown clock displayed on the top of the target's head.
The user can utilize their vitality to add, subtract, multiply, or divide the numbers at will, which in turn will either make the target's body become younger or older.
As such, they can technically make someone live forever but must always add years of life (at certain intervals) to maintain the target's young body. Now, while they can make anyone physically young again, they cannot do anything against any diseases (including cancer) or genetic defects that the target has. No matter how many years they add, they cannot cure them and may only be extending their target's suffering.
Moreover, if they make their target too young too quickly they will experience terrible pain as their body will literally breakdown back into a younger stage (like back to being a kid or even a baby). The same can happen if they age too quickly as their body will degenerate rapidly. In addition though, intelligence can be temporarily affected due to age (like becoming a baby), the target's memories and experiences will remain intact unless there are underlying health issues that interfere (such as mental disability due to genetic defects or physical malformations of the brain, etc.)
Drawbacks: mental and emotional fatigue from intense concentration and from the weight of playing god by deciding who lives or dies, physical exhaustion, and fainting if overuse severe enough.