Arthur is going on 24 this year. I've had people in recent years offer obscene amounts of money for him at fun shows, thinking he's no more than 7 years old and apparently well trained. I gallop, jump, barrel race, trail ride, and bareback on him, and he stays in the same condition as the 6-yr old in my avatar, who practically exercises himself!
Gain her trust by grooming, grazing, and spending time with her. Make your presence a positive thing, something to look forward to. Older horses often have better, calmer temperaments and a calm frame of mind that appears to a lot of people. Ride her for short periods of time at first, see what she likes (be it trail riding, arena work, or bareback walks around the pasture), and work from there. If you find that neither of you enjoy riding together, it's safe to retire her as a pasture puff.
If she seems sore or restricted when you start putting her through her paces on the ground or under saddle, try a senior or joint supplement and see if she improves.