Not too old! Especially for piano!
I currently teach nursing. Expectations are that I should be able to retire with a full pension in my mid-to-late fifties. I am thinking about pursuing a second career at that point, opening a piano studio in my home and teaching adult students. I figure that by that point, I’ll have spent approximately 30 years teaching adult students. Why could I not use the same principles, just different material?
I rather suspect that my students will fall into two or three main categories - those who wanted to play but never had the opportunity and just want to be able to sit down once a year and play Jingle Bells for a Christmas sing-along, those who used to play and now want to get back into it and just need a few lessons as a refresher, and those who maybe want to switch from another instrument.
My mother started taking cello lessons when she was about 50 or so. (She is already a pianist, organist, and singer.) She is almost 62 and still taking cello lessons and practicing. I say go for it!