Obviously there are many good reasons to learn or re-learn how to play the piano in your retirement years and into old age. The main one is simply because you have a desire to want to learn to play the piano. One of the more obscure reasons that might contribute to your decision to play the piano, is to maintain your fine motor skills as you age.
I have mentioned it before that playing the piano is good for your hands to keep your fingers and hands moving, but I have never really gone into any depth about it. I think that playing the piano is probably one of the most thorough ways of keeping your fingers moving well into old age.
There are several hobbies that are really great at keeping our hands moving. Sewing and doing all sorts of crafts are hobbies that immediately come to mind, but in my opinion, because of the somewhat repetitive nature involved, a person’s hand can stiff and aching from the activity. Gardening and woodwork or carpentry would also help keep a person’s hands and fingers moving.
Where I think that playing the piano has the advantage over all other hobbies, where keeping hands and fingers nimble, is that with playing the piano you can learn to use all ten fingers independently and actually strengthen the muscles that move these fingers. I think that this would be a real advantage as someone ages and could potentially help keep an elderly person independent or even semi-independent for longer.
I’m not saying that there might not be other activities that would help keep a person’s hand and fingers nimble, I’m saying that it is a fantastic way of maintaining flexible and usable joints in your fingers. Years ago I remember teaching a boy to play the piano, who had limited mobility, particularly in his left hand due to having a stroke as a baby, that after a year of lessons and practicing, this boy had almost perfect mobility in his hands and fingers. Playing the piano achieved more than what a physiotherapist (or physical therapist) could achieve and in less time.
It should not be the main reason why a person decides to learn to play the piano, but it definitely is a very good contributing reason to want to learn to play the piano, or to continue to play the piano well into old age.