You can hang on as long as your bank account and your body allow for it. Seriously, though, there's no upper age limit for pet sitters. Pet sitting can provide you with great exercise – from the dog walking, of course, but also from sprinting across the room to escape hungry, lunging dogs with no sense of ethics about their upcoming meal. In many cases, you can fit a pet sitting career around other work or personal commitments. Perhaps you want to augment your income with a home-based part-time job; or take classes during the afternoon, which still leaves you free during mornings and evenings. Basically, if you've got the client list, and your injuries aren't life threatening, you can keep going as long as you still enjoy it.