I find this an interesting question.
My DH is 51. My son-in-law is 25 and my son is 26. Without a shadow of a doubt, my DH could beat the shit out of both of them, probably even if it was two against one. Purely because DH is enormous and trained how to fight with his job. At some point though, the dynamic will shift and do a turn about. I wonder when that will be? Not 60, I don't think.
My FIL is 75 and my Dad is 81. The difference between them is STARK!
FIL can climb mountains, weight lift, walk nature trails etc and mentally he is as sharp as a pin. He does texting, whatsapp, ordering and selling stuff on-line. He drives a sports car. He does several long haul holidays every year.
My Dad, on the other hand, is wheelchair bound, needs carers 4 times a day and doesn't really understand how some things work any more.
Do I think of FIL as old? No.
Do I think of my Dad as old? Positively very elderly.