The way I read this there are two issues here.
(1) Not allowing under 19 into MLL unaccompanied. Legit legal reasons for that. They were probably right to deny entry, though the way they did it may not have been proper or respectful.
which leads into
(2) Some AC staff having bad attitudes towards young looking travellers. They try to deny other benefits (priority boarding, free checked bags) without bothering to check status or ticket class. Others have posted about similar problems (was cow one of them? ). This is a real problem, and is definitely worth a rant.