Being 60yo now, I would have been considered as being at the end of middle-aged in the year that I was born, but Australian men have a 20% longer life expectancy since then, so we get an extra 5 years before being considered old.
Per
@hank37w above, I have basically always seen anyone 15 years older as being old. As for feeling old, everything still works, though not as good as it used to and my 2027 begins with left knee replacement surgery that is also affecting my hip angle
My orthopaedic reckons my right knee has several years left in it, so hopefully surgery goes well and it helps me not feel old until until I officially get there.