Did the supermarket have the right to do this?

This happened a while ago now but it's stuck in my head. You see i was standing behind this man (in his late 50's at least) He was buying alcohol like vodka and stuff. A man that worked at this store said that as he was buying the girly drinks and the fact that he came in with his two daughters who must have been 16 that unless they produced an ID he could not buy the alcohol, as it was most likely that he was buying the alcohol for them and he was, they were having a party or something (yes i was very nosey and listened) But i learnt in citizenship although it's illegal to buy alcohol underage it's not illegal to drink underage the same with smoking. I think they said that as long as you have your parents permission or supervision, i can't remember then you are allowed whether it be a sip of wine or a bottle. The man that worked there said that it was supermarket policy. But their policy contradicts the law. They didn't have the right to not allow a man in his 50's who was clearly older than 21 to not buy alcohol even if it was for his daughters as it was a party, did they?? As HE was buying it. Do they have a right to do that?