Such a great thread!
I think physical / body age and the mental age aren't tightly related. What matters is the era you started and what we have now. I didn't grow up with siblings; only parents of retired grandparent age 50 and brothers age 30 which I had trouble relating to. Generations: my parents exactly the age of The Queen of England, and brothers the age of Prince Charles and his siblings.
A very important friend for a few years was 40 when I was 30. After tragedy in life and rehab, he rebuilt a new business which he had before, and got married. Won't speak again. He was a social conduit like many people I've needed.
I am scared of younger people. All the way through school, I saw anyone older as simple and stupid. Every girlfriend has been 4 years younger.
I'm approaching 40, and quite surprised I'm physically fit, quick, with a verve which people around in town or supermarket are melancholy, extremely slow, meandering, with no zest. I feel about 25, and dearly wish I'd been in the Army.
I do wish I could start again in the 1980s.
My Dad's only topic has always been: how terrible it feels to be old
Maybe my one remaining bro is feeling that. I might be worse, but I never give up and will never want to retire, but I have no choice, when nobody will employ me