Numerically, at what age do you consider someone old?

First, let me say that I don’t think the numerical number is a true indicator of “true age”; however, some people seem to be very hung up on the number. I’ve seen some 50+ seem to think they are old and can’t do things, other 50+ talk about how much more life they have to live – a lot! (I agree)

My mother, 65, is stronger and more active than some 40yr old people I know. She frowns when she is referred to as an “old person” – but agrees that she is getting older.

Where is the line, numerically? What do you consider old?