Hey, it could be worse, I suppose -- I had a flight attendant ask me to verify my age last year in an exit row. I was startled, to say the least.
I have also been told that I don't belong in the SP check-in line before - most recently by an agent in ANC who I guess did not see my PM tags and previous Sky Priority tags on my luggage, who asked if I was a FC passenger. I informed her that the line was reserved for FC passengers and Medallion members.
Stand your ground, even if it is a bit embarrassing. I may be young, but I have just as much of a right to be in the SC as any other paid member.