I think my friends know my age- I had parties to celebrate 50 and 55 and I expect I'll do one next summer when I turn 60. I don't feel my age, and I think I look younger than my age as long as my hair color is fresh... Wearing a bike helmet disguises a lot of wrinkles too.
Anyway- I don't make it a secret but I let my students puzzle over it. I usually tell them I'm about 157 years old and ask them, "don't you think I look pretty good for 157?"