I am currently riding a 30 year old 1/2 arab 1/2 QH. Most people cant tell how old he is. And there are days that I don't think he is that old. With an older horse as long as they don't have terrible arthritis and are in good shape they will let you know if they can handle riding. I don't ride mine hard any more but he still goes on 10 mile trail rides nothing hard just walk and maybe a little trot, nice simple rides no mountain trails.
Nutrition plays a key factor in keeping the weight on. Even if the horses teeth have be kept in good shape as they get up there in age it is best to switch their feed to something a little more dense in calories. Older horses don't have much tooth left and it is hard for them to chew the hay properly to get the needed nutrients. Also their gut doesn't function like it did when they were younger so they need food that is easier to digest.
I will use my old man for an example.
I used to feed just straight bermuda hay and he was plenty fat. About 2 years ago we noticed that he was losing his topline and not holding the weight so to his diet I added Equine Senior about 8 lbs a day since I was still feeding him hay. Last year when we had his teeth done I was sure that he would start packing the weight back on but he didn't. So I discussed with my vet (who I worked with) what Should we do. We decided to put him on Alfalfa Pellets (which I had to do slowly since he had never had alfalfa) and Equine Senior. This way he didn't have to chew his food to get the nutrients out of the hay.
Horses must chew their food properly to get the needed nutrients, Pellets are already broke down and senior is really made to just swallow.
So anyway now that he is on a diet that meets his needs he has filled back in and looks about 1/2 his age. And has a ton of energy, that is when I don't walk out with a saddle, he is quite a character.
So if they are kept in good condition and given the needed supplements and support, you should be ok
If they do have a little arthritis a little bute and a joint supplement will help and if it is in an area like the fetlock that can be wrapped with a polo when it is cold that will help also
Hope this Helped