A granddaughter asked me this this morning. These (my oldest) grandchildren are very interested in (my) old age, possibly because I'm one of the oldest people they're close to. When they were small, they used to tell me I was "very old, and going to die soon" (thanks, guys). Now the questions are different.
But I think the answer to this one (and the one I gave) is that I don't think I would know I was old if I didn't know my age. I certainly don't feel it. I ride, play with grandchildren (on the trampoline this morning), wear jeans, even flirt (yes. I do, just occasionally, get chatted up. Hard to believe, I know. But you neve forget how to flirt. Experto crede). We are obsessed by age. Big O birthdays, retirement age, pensions, nice little offers of funeral plan packages or retirement homes coming in the post. You can't get away from it.
I have an actress friend who has told no one her age; not even her son. And I think that must be so liberating. Too late for me now, but good luck to her. She really can be as young as she feels (awful cliche, but it fits).