Both of my riding horses are 16 and 17. While I would like for them to be younger, I am still taking them on 2-5 hour rides regularly. They both seem to be in great health. The 16 yr. Old just had a baby (not my doing, but I am happy to have him!).
So basically, ride them as long as you can, and just be mindful of their age if they start having issues. But right now, at 16 and 17 I am not even mindful of their age, because they seem absolutely fine.
I have had horses rideable until they died (of colic) at ages 24 and 26.
Even my vet says to keep riding them, even the ones in their mid 20's, because when you quit riding them is when they loose muscle ton and their health deteriorates. I only had one horse I had to quit riding for a few years before he died. I put him down at age 32 because several years before he had slipped and injured his back. So he wasn't rideable after that, and his arthritis issues steadily got worse, so I finally decided it was time to let him go. But short of that, if they are healthy, then exercise is good for them. If they start getting arthritis issues, then you will know it is time to start taking it easier on them.