Martha I think it depends a bit on how fit you are, how determined you are, and what you want to do. One of the owners at my yard must have been in his late 40s/ early 50s when he learned. He has his own horse and happily hacks out on him even though he's a bit of a handful. Horse and owner have a lovely relationship and enjoy spending time together.
If you want to compete at the Olympics it's very unlikely. If you want to enjoy hacking out that's absolutely achievable. If you eventually wanted to get your own horse and do riding club competitions that's definitely achievable, but would just take more work and more lessons.