You are lucky to have a young face. When you get older you will understand.
People laugh at me when I tell them I'm 21 because I have a young looking baby type face. Are they laughing to be rude or are they just surprised? I still prefer to enjoy family friendly stuff it's what I feel comfortable in.
They are probably just suprised. You should feel blessed!
Doesn't matter why they do but most chances are they are jealous and cant believe it. Anyway, who really cares? Enjoy it while it lasts, as nothing in life lasts forever. I had the same problem when I was 21 and I was a US Federal Officer with full credentials and it lasted til I was in my early 30s but now in my 60s, its a memory and a age I wish I could return to.
So next the time someone laughs, tell them to eat their heart out and you just need to accept peoples reactions and enjoy it. Good luck