Unnatural Aging

Unnatural aging can be caused by several things, including ghosts and every magical cause on DMG 13.

Effect on age categories

Unnatural aging does not increase maturity. It is a withering effect, reducing lifespan.¹⁾ Therefore, when checking age category on DMG 13:

For the purpose of determining cumulative positive modifiers, use the character's real age (without unnatural aging).
For the purpose of determining cumulative negative modifiers, use the character's unnatural age (real page plus unnatural aging).

¹⁾

DMG 13: “loss of life span”. For a better idea of the intent here, refer to OD&D Vol 2. pg. 35: “Staff of Withering: A Staff which adds nothing to hit probability, but when a hit is scored it scores one die of damage and ages the creature struck by ten years. (This is not to say it matures it, but rather it shortens the life span by ten years.) A man struck four times thusly will be doddering, an animal dead of old age, and so on.”