//Age shouldn't really come into it//
yes it should. A film rating system is designated to classify films with regard to suitability for audiences in terms of issues such as sex, violence, substance abuse, profanity, impudence or other types of mature content - this is designed to help parents decide whether a movie is suitable for their child.
From your posts, I assume that you are not an adult, so therefore age DOES come into it. If you were to buy a 15 classified or 18 movie, when you are not, the seller could face a £5,000 fine and potentially up to six months imprisonment.