Odds of Hanging On
The question isn't whether you'll be able to hang on as much as whether you'll want to continue living this kind of life. It's typically a younger person's game, especially if the celebrity is really into the nightlife. This can be a stepping stone while making contacts and learning the industry from the inside out, and even the nicest famous person can make life a living nightmare.
Once you've had your fill of the abuse, the long hours, the lack of freedom, and the strain on your personal relationships, you'll be out of there. Of course, there are probably some lifers out there. If you come across any sixty-year-old personal assistants, please pass along our condolences.