I think old is as unique as each individual. To me it is relative to how I feel at any given time and also with the passage of time. The older I become chronologically, the further "out" an age of being old becomes to me.
Remember the time of not being able to "trust" anyone over 30?
I definitely do not see my parents as "old" even though they are 71 and 73. Their zest for life and their energy, stamina and never ending activities often times amaze me. On the other hand I have known people younger than them become very old in a hurry (usually after retirement). It's like some of them are just waiting out their time to die instead of taking full advantage of the time they have left. To me, that is very sad to see.
I have a grandmother who just turned 93 last week. She still lives by herself, goes out to play cards with the girls a couple of times a week, has recently learned how to do Country Line Dancing and makes the trip to the beauty parlour to have her hair done once a week. She's 93! She should be old, yes? I don't see her as that at all.
Personally, I have some days where I still feel 25 and other days when I feel 105. The mental outlook always remains at a pretty constant 25, but the physical realities sometimes pushes the envelope to the 105 mark.
It is then that I always try to affirm that age really is just mind over matter. If you don't mind, then it doesn't matter.