I am 73, have MS, and I still ride horses. I also do NOT have much energy, I rarely last more than 30 minutes in the saddle during my weekly lesson on a real horse, and I only have one lesson a week, weather permitting. There is NO WAY that I will ever have enough energy to go to the gym, join an exercise class, life weights or do Pilates even once a week.
The only outside exercise I do for riding is once a week usually less than 30 minutes on my riding simulator the Home Horse https://www.homehorse.com. With the bridle extension it is $1,090.00 US (cheaper without the bridle extension.)
I have found that using my Home Horse really helps with me finding my place in the saddle, and that the Home Horse serves to graphically point out how my position faults affect my balance in the saddle. I have no fear of falling off my Home Horse even when it lurches alarmingly under me, which gives my body “room” with which to learn how to adapt my elderly body to the movement under my seat.
On the HH I can isolate using just one muscle to exercise it and get it stronger, alternating which leg I exercise so both get stronger. I can experiment with doing exercises for two muscles.
The balancing part of riding the HH also helps me a great deal on a live horse. I put a long bubble balance on the HH platform so my spotter can tell me immediately when I get off balance. If I bow my head down to look at my feet or my hands the HH lurches drastically on its “forehand”. It rocks back just as drastically when I sit too far back. If I move my hand to one side the HH reacts to this. I can feel the difference between an indirect rein in front of the withers and an indirect rein to the rear of the withers, yes the difference is subtle but I can feel the difference in how the HH moves under me.
Riding my HH keeps me fit enough so that my riding muscles do not get sore even if fate determines that I do not get to ride a horse for several weeks. Using the bridle extension helps remind me to keep opening and closing my elbows to keep good contact.
You could even encourage your elderly riders to buy their own Home Horse to use at home. At today’s costs of lessons &/or homework rides the HH will quickly pay for itself.