Not knowing the OP, perhaps he looks under the age of 18 and the LEO was only verifying age. Kinda like the card everyone under 40 for smokes now.
Had the OP been lets say a minor with a handgun, or even not a good guy but a bad guy the LEO was doing some investigative work to make sure the individual who has a weapon is legal. Can you fault an LEO for following up on something like this? There are a lot of What if's that can come in to play on this scenario and we don't really need to get into them here, but just sayin I don't have a problem with an LEO following up on a hunch like this, it was less than 2 minutes out of the OP's time for an officer to check to make sure a weapon was being carried legally.
I think this is the case, because I have been told on many occasions that I look much younger than I am. I was once told by a lady that she didn't know that you could be sixteen and be an EMT. Most people assume me to be 16/17 until I provide the DL. Especially when I have to run and get cigarettes or something. So, I don't blame him a bit. I was just happy that it all went smooth and he got a good chuckle out of it.
To Dgiancola: Absolutely not.