I agree there doesn't seem to be senior specific foods. Well, for the first 10 years I've fed Baby a seed mix + fresh, cooked bread, whole wheat bread, rice, pasta, etc. I still feed the same fresh foods. I think one of the most important things I can feed him now as he's getting older, is veggies. I really wish I introduced them when I first got him in 2002. Around 2013 I switched to pellets, until finally a good quality seed mix. Now, he gets a mixture of seeds and pellets. I've heard conflicting stories about pellets and seeds, so I just feed him both. I think a probiotic is something to look into.
Another thing these days, is just ensuring the foods he eats are easier for him.