I currently work in an alcohol shop(though just 1 day per week) and I must admit that it took me a while work out the system of when,why, how to deal with potential underaged and then I still find it quite tricky.
Why the under 21 rule? Well lets see what happens without it. When I first started work, I had it in my mind that what I have to do is beware not to serve alcohol to someone under 18. Thats the case anyway but if someone comes over that looks suspicious and hasnt got an ID, and you tell him "well,you dont look 18+ to me", there will be a great chance that things would end up in a 20 min pointless arguing with him/her trying to convince me otherwise. In that way, they basically accept it as it all depends on me and Im trying to be an arse towards them. I must point out here that women are worse and I will tell you why(not to be accused of being a sexist)...1) their nowadays overuse of make-up makes it a complete mystery to guess their age, not to mention that certain few refuse to disclose it...2) the kind of young women going for alcohol all the time are more likely to start arguing than men.
So here is how we benefit from the under 21 rule. Its a rather subtle thing but the rule allows us(the sales assistants) to change our awareness from looking whether people are 18+ to looking if they are 21+. This is way safer than 18 because even if you fail to judge someone correctly, you would, in 99% of the cases, fall within the 18-21 range which is still ok. Argumentwise, if someone goes for "Im not 21 yet but Im definitely 18"(without being able to prove it), I can always go "Im sorry but BY LAW I am entitled to refuse to serve you since you DO NOT LOOK like being 21 or more". The words with capital letters simply work wonders against people ready to stay there and argue.
I particularly find funny, the people that come up with "if you dont serve me here,I will go elsewhere" or "Pls serve me! I wont tell anyone!". From what I have seen, only about 1-2 people out of 10 would try to argue/negotiate and they truly are over 18. They would be the ones to quickly go back home to get ID etc, the rest are really under 18 and are aware that they got no option but to try their luck and their actual lack of respect towards the law ,and thinking that I would break it for them, truly reveals their (at least mental) age. Most adults show they deserve their age by happily taking an ID out of their wallet, knowing what sort of risk is placed on you if you fail. So ladies and gents, whoever deals with such kind of job, ID the kiddies to their bones and feel no regret!
ps - didnt mention the ciggies cauz the 21 rule applies there too while you only gotta be careful for people who are less than 16.