How old do you need to be to be considered elderly?

I was recently thinking, and this question occured to me. Where do I draw the line before I consider a person old?

For me, it's a difficult question to answer. It appears that the older we grow ourselves, the further away we consider this line defining "old age". Is this in-denial, a way for people to avoid the fact that they are no longer as young as they would wish to be? Or is it that we just never consider ourselves to be elderly? As I'm still relatively young, this isn't a problem for me, yet when I talk to other people like relatives, that line of old age always seems to increase the older they get themselves.

Then there's the problem of physical age and mental age. I've met 70 year-olds who still act 20, and 20 year-olds who act 70. The phrase "young at heart" springs to mind here.

My question is, at what age is this elderly line for you? Beyond what age would you consider somebody to be old? And do you usually judge this based on how old they appear physically or how old they act mentally?