Ride World Wide. I went with them to Argentina - Estancia La Rosita - amazing value - we did the 8 day estancia to estancia ride - long hours in the saddle but amazing - bonus is that the wildlife is not trying to eat you ....... and Okavango Horse Safaris - v v expensive - lots of saving and selling stuff to finance it but a real trip of a lifetime - brilliant horses -
both require a reasonable riding background especially OHS as you need to be able to move at speed if necessary but I know of at least a couple of over 70s that go regularly.
You probably want real riding, so be careful. I went with a friend on a riding holiday in Spain and it turned out to be 3 hours a day sitting on a plodding pony and the rest drinking tequilla, etc. Not that I did the latter, it was a horrible disappointment. I've often heard riding school owners complain about the number of people who turn up thinking they can ride without the foggiest that knowing how to ride actually means!