The first person to use the Comb of Youth and Vitality removes one year of age for every brush stroke. This person instantly feels themselves getting younger and more energetic, and so can usually determine what is happening and stop before, say, becoming a child.
You're going to want to write down how many years the first person removes, though.
All subsequent users of the comb age that many years for each brush stroke. So, if the first person removed twenty years of age, all subsequent users become twenty years older for each brush stroke. Brush your hair in four strokes, and you're now eighty years older (and likely dead, if you're a normal human). If your players like to tinker, it's likely at least a couple of them will try the comb, hoping that the effect will somehow skip a person or whatever.
After the initial use, the enervation effect lasts for a number of years equal to the years of age stolen. So, if you lost twenty years of age, all subsequent users will gain twenty years per stroke for the next twenty years. Then the comb "resets" — probably after a group of concerned do-gooders locked it away somewhere and it has been forgotten.
Just in time for another to use the comb.