How you can tell that you are getting old: When someone tells you that you don't look a day over 50 and it is sincere compliment (but still intended to flatter you).
One other way: When you are over 50 and someone tells you that you look 25: it is not a sincere compliment but just intended to flatter you. For over 60, it is 35.
My team lead joked on my birthday that it was an anniversary of my 19th birthday. He is trying to avoid any age discrimination. He's 27, so just a baby. I appreciate his tact, but you don't get this much experience at 19. 4 years ago I had a female manager of a different department tell me I looked about 50, so that made me happy. In the long run, I don't care what I look like, that was eliminated when my dad told me I wasn't pretty when I was a child. He had all the tact of a dump truck. To me, pretty is a chance of genetics. I'm smart, sassy, and witty, and those are MY doing, not genetics or surgery. When someone, especially male tells me I'm pretty I don't believe them. If they want to get on my good side engage me in wits, compliment my reasoning, talk about important things.