I understand what you are feeling and in a minute I will tell you why, but the short answer is that age is just a number. People have young horses that have health issues and can't be ridden while many of us have older horses that are still going strong,
When I bought my mare, the pre purchase vet I used aged her as 13-15. About 6 months after I bought her, I had another vet come out to float her teeth and he told me she was atleast 23-24.
I was devastated, i felt like I had lost almost 10 years of her life.
The event that snapped me out of it was when my friend's 6 year old horse colicked and died.
Age doesn't mean anything.
As long as your horse is sound and healthy, you can continue to ride her. I ride my mare 2-3 times a week out on the trail and in the arena. We run barrels. She does judged trail rides. She is happiest when she has a job to do and being in regular work keeps her stiffness to a minimum.
Now, I would never make her jump or do endurance because I know that would be asking too much of her joints...but the type of riding we do is encouraged by my vet to keep her joints moving.
I have her on a joint supplement and also B-L pellets (all natural painkiller) to keep her comfortable, especially in the winter months.
But she is happy, never refuses a meal, and still has more go than whoa. Anyone that rides her can't believe she is in her mid twenties. My girl loves to go!
There is a 34 year old QH at the barn that still gives the occasional lesson and trail ride and has yet to take an unsound step.
So, don't spend too much time worrying about when you should retire her. He age has nothing to do with it. Retire her when your vet suggests she shouldn't be ridden anymore. Forget her age. It is just a number. :)