Under 21 years old ?

I went to Tesco with my friend today who is a Japanese. I bought cider there . My friend was going to buy a bottle of wine but she couldn't do it because she looks under 21. She asked me to help but I could only say that she was definitely over 21. Of course, I can not prove her age even if I insist that she is over 21 years old. She had no choice but to leave there without buying it. On the other hand, my checker asked me whether I've got ID when I bought my cider. I thought that she was joking, so I responded that I was not young with my smile and she said that I wasn't old was I. I bought cider without a problem but my friend could not even though she took an hour to walk from her place. What a pity!!