Not someone but something.
For me, there are three things that would make me stop:
1: I lose too much hair. Either just on top or going completely bald. I won't wear a wig.
2: I become fat. If my gut starts to hang so low or gets so large I can't see my feet.
3: I can't dress myself the way I like. If I have to trade in my short shorts and mini skirts for slacks and pant suits I might as well wear jorts.
It may sound vain but that's the way it is.
So what about you?