The laws are slightly stupid and arbitrary and dispense with common sense, but they are nonetheless the laws, and it's ridiculous to expect the staff to break those rules. Babies aren't allowed in bookmakers either because they're under-18... even newborns who clearly have no idea where they are... the law is what it is and staff can be fined up to £5000 and / or jailed for 6 months (in Scotland, longer in England I think) for failing to immediately tell anyone who brings a child in to leave. It's literally a mad scramble to get them the **** out. A lot of the parents get pretty pissed off. Frankly, it's tough ****.
The laws apply right across the board even where the "potential adult material" isn't even available to view at the time. For example, when I went to buy GTA5 (a game, rated 18) I had my four year old daughter with me... she likes to carry out the transaction is shops for fun (give them the item, hand over the money, get the item in a bag) and on that occasion, the guy serving after taking the money had to say "I know this is totally ridiculous, but I can't hand her the bag, I have to give it to you and then you can hand it to her..." because it's against the law for him to hand over an 18-rated item to someone underage... even though it was obviously for me, not her. It's just rules and regulations. Stupid ones... but, blame the policy makers, not the wageslaves.
I personally am against age rating as anything other than a guideline, anyway. IMO if a parent thinks their 13 year old is OK to go with them to see an 18 rated horror film, and the 13 year old also wants to, then they should be allowed to do so.
The original "Total Recall" (with gool ol' Arnie!) was one of my favourite films when I was about 9...