I have a 18 year old mare and the pony I borrowed was 23 years old, I gave them as much good quality hay she want's. High in protein and low in sugar.
I would feed them a senior feed or a stud feed, depending on the analysis of the hay. I often soak the feed in water, just to make sure they get some extra water.
I also give them multivitamins.
And, when my mare's not in foal, I give her a joint supplement called Congrogen and it's a mix of hyaluronic acid, chondroitin sulfate, glucosamine sulphate and manganese sulphate - work miracles on old horses. (lol not sure I got the translation right...)
Make sure to give the horse extra proteins right before it starts to shed it's winter coat.
I think old horses need some light exercise every day. Perhaps a little walk for about 30 minutes.
Check the teeth often, at least once a year. Mine get her teeth checked more often then so, but we rarely do anything with them, just get a checkup to see if everything is on order.
My mare, (230 days pregnant.)
My old pony, 23 years in these pictures. He competed in the biggest competitions you could here in Sweden until he was 25.
Stopped competing at age 26 and nowadays he's keeping the youngsters company.