This is a good one. It's not very scientific, but it has the benefit of shock value. Here's how it works.
Have you ever suddenly surprisingly found yourself in front of a mirror (or a plate-glass window), glanced at the image, and seen—your mother?! (ot your father?) Seen that you looked older than you thought you were? Wasn't that a shock? Didn't a little of the spring go out of your step? Didn't you make a mental note to get that checkup you've been putting off? Didn't you think for just a moment about liposuction or even a facelift?
Or have you ever come upon a mirror you weren't expecting and seen the reflection of a youthful, smiling face, and a slender figure, then, only a second or two later, realized it was your own. You looked thin and stylish. Wasn't that a pleasant surprise? Didn't you decide to treat yourself to that new lipstick you thought was maybe too bright for an "old: lady" like you? Didn't you think that maybe tonight that sexy nightie you put away should come out of its box? Or that maybe it really wasn't] too late to take up tennis?
The power of the mirror and what we see in it is one way we determine how old we would be if we didn't know how old we were.