I don't think old exists. In the physical sense, yes you'll get grey hairs, your body will physically age and get slightly weaker over time perhaps but I personally think it's a state of mind.
Some people say that actually once you hit retirement age then that's when life really begins as you have no added stress through work, hopefully have saved a decent money for a good pension, can travel around and have more time for hobbies.
I'm 29 but probably still act like a 20 year old... some say 29 is getting old... crazy really.
It is clicheed but you really are as old as you feel. Live how you want to live and don't feel constricted by life stages and what you're meant to earn by a certain age or when you're meant to be married etc. All happens organically.