The first thing to look at in growing hay would be where is the hay going to be grown? What region of the country and what climate is the first thing to determine, and what hay would best be grown in that region. It might be a good idea to have your soil tested and to speak with local farmers and your feed and ranch store to see what would grow best in your area.
Farm and ranch stores will most likely sell the best seed that is suited for that area.
Alfalfa hay is very rich in protein so you would want to be careful feeding too much to your older horse. If he is doing well on the grass mix that is probably fine. Most times staright alfalfa hay is fed in small portions even to younger horses. It is very rich and can give horses diarrhea. On the racetrack we would feed straight timothy hay and a “flake” of alfalfa in the morning and matybe at night, but not much more than that.