Is it kind of a diss if someone thinks you're younger than you are?

I'm a local at a bar and one of the bartenders, who over time has become a friend of mine, got off work and him and his girl friend sat down with me on the patio. He's 23, she's 22. At one point in our conversation they were telling a story involving an "old" guy that they thought was silly for trying to hang out with their friend group. Eventually they reveal that the guy was 40. I start laughing my ass off and they're confused. I tell them I'm 38. They stare at me with disbelief and said they expected I was around 27 years old.

Sure it's a little bit of a compliment when someone guesses you're younger than you are, but... what does it really mean? Do they view me as if I'm a "kid their age"? Does it mean they see me as immature for a 38 year old? Is it bad that I associate with people in their early 20s?