I think the point here is that most of the time we don't feel any older at 70 than we did when we were 40,
Of course, sometimes our bodies remind us that we are neither 20 nor 40 anymore.
But on the whole, unless it is one of the days where our knees hurt, or whatever it is that hurts and reminds of our age, we don't think about it, so it comes as a shock if someone calls us old.
The other point is that we have lived our entire adult lives in a time that glorifies young adulthood. We all know men and women who try very hard to look at least twenty years younger than they are, and probably none of us enjoy catching sight of ourselves in a window or mirror and wondering who that old woman is, or thinking "She looks just like Grannie! Oh, she is me!"
Perhaps we should try to remember that the alternative to old age in anyone' s life cycle is not youth, but death.
That alters the perspective a bit, doesn't it?