FWIW, I went to a seminar on the care of the Senior Horse at EquineAffaire, and the vet who was giving the talk said that they don't consider that - for most horses, YMMV - "Senior" even begins until 25 these days. All the improvements in nutrition, hoof care, parasite control have really lengthened the active lifespan.
The other thing that she, and every other vet and trainer I've talked to, says is that the *best* thing you can do for older horse is ride them frequently. Not manically, but very regularly.
My boy is 18, and he might as well be 10. It takes him a little while to warm up some days, but once he's warmed up, he's good to go. His mind is sharp, he's enthusiastic, he's energetic.