I think "old" is a state of mind. I understand physical limitations, but those can happen at any age. Hubby is going in for a hip replacement this month, and he's not 60 yet. Dad (FIL, but he's been "Dad" for almost 30 years and I'm not changing now!) will be 80 in October; we put him on 3 wheels back in 2012, on a Springer that hubby put a Frankenstein trike kit on (side note. don't do it!) He's put maybe 500 miles on the trike since then. This April, we put him on a 2024 RTL. He's got almost 1500 miles on it already. He says he felt old on the trike, but is enjoying feeling young again on the tricycle. He walks down here daily, weather permitting, and puts in some miles on the treadmill when weather doesn't permit. My Mom, on the other hand, is physically okay but losing her mental faculties. She's old at 77 and has been for a few years now. My Dad passed at 90, went to work every day up until his last hospital stay. Stayed active, still drove, planned trips, would debate you on any subject just for fun. MIL passed this March, having been old for 10 or more years, physically, but mentally still sharp. Some days, I'm old. Really old. Some days, I'm not sure that the birthdate on my drivers license is correct... no way I'm that old!
I don't think there's any official age that says "you're old now". We all have to age, but we don't have to grow up!