Odds of Hanging On
A sitter can typically hang on as long as their bank account and body allow for it. Seriously, though, there's no upper age limit for pet sitters. Pet sitting can be a great way to get some 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, sitters can fit their career around other work or personal commitments, so that also helps them hang on. Augmenting income with a home-based part-time job, or taking classes during the afternoon still leaves sitters free during mornings and evenings. Basically, if a sitter has the client list, and their injuries aren't life threatening, they can keep going as long as they still enjoy it.