I've seen your edit and it is interesting. I work in an off-licence and we have a Challenge 25 policy in place (voluntarily, not because we're on special measures or anything). If you're not sure what this is, it means that if I think a customer is under 25 years of age I have to ask for proof that they're over 18, and if they can't provide satisfactory ID, then I have to refuse the sale. (If I think someone is buying alcohol with the intention of supplying to minors, I can refuse the sale anyway, but that's a separate issue.) The policy is displayed in at least three places that I can think of off-hand, so that it doesn't come as a shock to the customer.

What you were doing is legal and if the business has a different policy, surely it's in their own interest to display this policy somewhere (the menu would be a start). After all, they want your repeat custom. I do agree with you that how can we teach our children responsible drinking if they're not allowed one drink while out with the family.