Unfortunately, in Britain, you have to be fifteen to watch a film rated 15 in cinemas, even if you're accompanied by a parent. This also applies to films rated 18, for which you are likely to be asked for some form of ID if the staff think you look under eighteen. These laws are set by the BBFC (British Board of Film Classification)
Exceptions
The only exceptions to this rule are films rated 12A. This rating causes a lot of confusion among children and parents alike, as it is different to the other age-restricted ratings.
To see a movie rated 12A in cinemas, you must be twelve or older, or have somebody over 18 with you. All responsibilty for letting an under-twelve view a 12A film lies with the accompanying adult, and it is up to them to decide whether the film is suitable.
Other Countries
In Ireland, there is also a 15A rating for cinematic releases. This operates on the same principle as the 12A rating: If you're under 15, you must be accompanied by an adult.
In the United States, you are allowed to watch a film rated anything up to R (Restricted) if you are under 17, providing you have an adult with you. Movies rated NC-17 are the only exception; you must be 17 or older, regardless of whether you're viewing the film with an adult.