I think generally you have to be old enough to buy it to be able to sell it, ie 16 for cigarettes, 18 for alcohol etc. Not sure how this works in supermarkets though.
in supermarkets, under 18's have to ask an older member of staff to see them sell the alcohol! bit silly but hey it s the law.. but I think that you are all right in that you have to be 18 to serve behind a bar.