The average age when they quit is 25.
Mainly because they just can't keep up with the younger counterparts that are new to the business and are the novelty with the customers.
Of course as in any other business, you will find exceptions, like the ones Sampson says.
I know 2-3 girls that go into 30 plus and still working, but one thing you will not find at the normal city clubs (big cities, not the ones in the border area), are girls going 35 and up past 40 years.