thats the whole point delefia - there is no real definitive way to judge and the shops are at the mercy of the individual to make that judgement - and when that judgement is coming from an 18 year old trainee, you can't expect perfect results.
you cannot teach someone how to tell, you just have to hope they have a vague idea.
i would think its harder to tell with women too, with all the makeup and clothes you see 16 year olds wearing these days.
staff are told if its not obvious TO THEM then to ask - no point in asking if its blatantly obvious the person is older is there?
i have had it mayself and it is indeed very annoying, especially when the person serving is only about 18 themselves, but there is no point in getting upset about it - and claiming discrimination is ridiculous - are you smelling a lawsuit brewing here?
is that what all this is about - you want compensation?
just get yourself an id card, and be flattered that you look so youthful (and stop getting the botox, lol)