How Old Is Old?

We have an “aging” population. Individually, with each passing day, each of us inevitably gets a day “older”. But at what point do we become “old”? While each of us can almost certainly think of someone we would characterize as “old”, is there ever a point at which we think of ourselves as “old”? How do we make those judgments? What difference does it make when someone is thought to be “old”?