I know this is very subjective, and I think it definitely changes as you get older yourself, but what age do you consider someone old?
On Sunday I (44m), went to see my dad as it was his 83rd birthday. He is very fit and healthy and completely mobile.
When I got there his older brother was there, who is 89 and incredibly fit. He was telling me how every morning he hangs from a pull up bar for 70 seconds, and then does 50 seconds of knees up running on the spot. He worked as a bricklayer 5 days a week until he was 87!
Anyway digressing from the point now I would say someone is old at 85 or above, but if you’d asked me 20 years ago I would probably have said 75 or above.