As long as your horse is sound and healthy I don't see any problem continuing to ride him.
As a teenager I first saw a little palomino gelding and fell in love on the spot. Six months later that same gelding was dropped off at my parents farm to go for slaughter, he was so lame he could not walk ten feet. We took a chance and bought him for current meat price. Working with vets and a great farrier we got him sound again. At the time he was already in his teens. This little horse was rode all the time after that. I did open shows, gymkhana, worked cattle, taught children how to ride, you name it he did it.
About ten years later i thought I would do him a favor and retire him, he deserved it in my eyes. No longer having a job to do he actually went downhill and |I really believe it was depression. Old Goldie went back into full time work as my then son's horse and also in the feedlot. He became happy healthy and sound again. I have photos of him barrel racing at age 32. I competed in a ranch rodeo on him at 33.
Goldie was rode up until he died, not from age but from a tragic vehicle accident at 36 years of age. He never looked or acted his age as long as we kept riding him.
So just let your horse tell you when it is time to retire.