I'm in my early 70s but talking to some 40/50 year olds they sound and act as if they are old enough to be my mother. I didn't retire until I was 70 and then only because of family circumstances outside my control. I'm a regular gym user, am learning a second foreign language and do volunteer work several days a week. Anyone who calls me old can fuck off.
(Oh, and I was out jogging with my 47 year old daughter the other week and as we approached a post box she said "excuse me" took a run and leapfrogged it).
It's a mental attitude that runs in the family.
We are still both trying to decide what we will do when we grow up 
Some people, however, are born middle aged.