oh silly girls! I am 59 and I ride better now than I ever have. It is about base fitness and taking care of yourself, and not about your chronological age.
I am winding down from 6-8 horses to 2 only because I do get tired doing all of the farm/barn labor, and I am trying to branch out from just eventing to include more social activities such as fox hunting. Did a 2* in 2013 and the only reason I am not eventing at the ULs now is because I have young/green horses that are not ready for that.
So there is really no planning that can be made at this point. When your body gives out then you will wind down. Otherwise, just ignore people when they make those admiring comments such as, “Wow, you are amazing!” and your realize they are marveling at your still riding at a ripe age. Phooey, all a bunch of Hooey.
We should stop thinking about age and just kick on. That being said, I do think about my age a lot only because I do feel more tired after laboring on the farm - but when I am in the tack, it is the furthest from my mind.