If you rode at this stables, and took lessons there consistently, I would just ask. However, since you ride at a different stables, I would think that you should handle asking just like applying for a job. Age is a huge factor, along with liability risks and safety. Saying your an intermediate rider does nothing unless she has seen your ride / handle horses.
If you are thinking more towards working off payment for a lease horse, that is a bit different, and in that case I would go in asking about available horses. Some stables don't let people work off lease, but that's something you would have to find out.