I know some people won't understand or sympathize. I get it, but ask you to look at it from my point of view.
I'll be 42 in a few days and I do not look it. When people guess, they never put me over 33, and they say that with doubt. The shock on their faces to find out I'm in my 40s is both ammusing and disheartening.
While it can be helpful for carnival games or shock value, it can be pretty annoying and even cause problems in other area of my life.
For starters, dating.
Women my age think I'm too young for them, and therefore most likely immature. The one's that do want to give me a chance, and actively pursue me, are in their 20s. Some guys may want this, but I don't get the attraction. I'm old enough to be their dad. I want kids, but I don't want to raise my girlfriend. Such a large age difference is, to me, uncomfortable. I want to get married and have a family. Not go to clubs and whatever else girls in their 20s do.
Another problem is professionally. It takes a lot more work to get people to take you seriously when they think you're younger. I have done jobs (back when I worked in HVAC) where my assist was younger than me, but because he looked older, the customer would direct questions to them. Its a small thing, but at the same time it isn't. We all want to be seen as knowledgable in our fields, and that feels like a slight. 
Growing up my mom always told me I'd be happy as I got older that I have such good genes. Well, I'm still waiting. Maybe when I'm in my late 50s-early 60s and people think I'm in my 40s.....