I wouldnt exactly say its negetivity directed at them because they're underage. Its usually because they're trying to slither their way through the Age Verification system or being generally retarded on the forums.
Not only that, There are liabilities to be considored. being 18+ Makes yourself liable for your own actions.
I was able to participate in airsoft games when i was 16 because I had proven myself to be mature and knowlegable about the sport. but I wasnt really granted any special priveledges beyond that.