Age is not a number. It's a state of mind. I have met many folks who seem very young, but by their acutal age, they would be considered quite old, and vice versa. Some people are very active and bubbly past their 70's and others wind up in wheelchairs or bed ridden. Your health has quite a bit to do with how well you age. If you are healthy and active, I believe you have a better chance of aging well.
The average life expectancy of men and women is slightly different, but for general conversation, that would be about 80 years old. There is a significant number of the population that is older and quite active. I would consider these people young for their age.
The US Government now has the retirement age at 67 years old. They have concluded that once you achieve that age, you are no longer viably employable and you are eligible to collect social security payments.
AARP has used the age of 50 as the minimum age required for membership. However, I think this is mainly to increase their membership and that this is the age that you start pondering your retirement and issues regarding retirement and aging start to become of some importance to you.
In conclusion, you are only as old as you feel, but I would actually peg the official qualification as being old at somewhere around 70 years old. In my opinion, this would be the winter of your life. But I would encourage everyone to prove me wrong.