Old?

In my youth the world was my oyster...what ever that means. But when does someone officially become to old? Does 'old' affiliate itself with a number, a disease, failed marriages, wrinkles or none of the above? I find myself rambling on about this at the moment because my best girlfriend in the world just turned 70 and she is a mountain climber and a machinest by trade. In the fall she will be attending school to earn her first degree. So I guess I am left wondering, am I old? I can't do half the physical activities she can.......