Same thing happened to me at JFK Club World check in.
I was 16 at the time travelling in Club World and I was told,
Sir, this is business class. Economy class is over there.
I politely replied that I was travelling in business class. He wanted to see my ticket. How odd that he didn't ask this to the 45 year old man in a suit in front of me!
I just took it as a compliment.
On boarding, I went upstairs and asked if the crew wanted to see my boarding pass - alas they said, "No, we believe that if you managed to find your way upstairs your probably in business class."
They were a very good crew.