Age is just a number! I believe with proper maintenance, a good fitness & nutrition plan, and a healthy dose of good luck (and maybe Irish genetics), these goals can be within reach for anyone who sets their mind to it.
I believe that motion is the best medicine. While we certainly showed a lot in our early days, I tried to be conservative with how much I jumped Ben at home and always gave him time off after shows to recover. He always got iced/bandaged/etc. after jump schools. We were extremely rigorous with his fitness regimen, which included lots of walking up and down hills (when possible) in addition to galloping. At horse shows when he couldn’t be turned out, I would walk and graze him for hours each day so he could move his muscles. I used to take him to week-long jumper shows and only do a handful of classes because I didn’t want to jump him multiple days in a row. Obviously having financial support from my parents when I was competing so much helped make all this possible and gave me options many other folks didn’t have (like the luxury to go to a jumper show and just hack on half the days), and that I don’t necessarily have today! Even in his later years, I never let him just sit in a stall if I could help it. He got lots of turnouts and handwalks, etc. These days he sleeps in a stall with a run, but is out all day.
You know your horse best, and I think if you listen to her, support her, and give her a shot, she will take you there if she can 
And I can also sympathize with getting cobbled back together! I recently shattered my ankle snowboarding and just had surgery to put it back together. I’m a little nervous about it impacting my ability to ride, but trying to stay optimistic.
Thank you for the kind words!