Computer and video games are age-rated according to the PEGI (Pan-European Games Information) system, and all packaging is visibly marked, front and back, with the traffic light age bands 3, 7, 12, 16 and 18. The square symbols on the linked page are examples of the PEGI ratings.
Where necessary, icons found on the back of the box indicate the content to be found in the game: drugs, bad language, sexual, violence, discrimination or fear. These are called game descriptors and are also detailed on the PEGI website.
Everyone in the UK adheres to this system, including the major console manufacturers such as Nintendo, Microsoft and Sony, as well as by publishers, developers and retailers.
Note that there are also historic ratings under the Video Recordings Act of 1984 where some games were referred to the BBFC (British Board of Film Classification) and may have a BBFC rating, usually 15 or 18. Additionally, older games will have the black and white versions of the PEGI badges with a plus next to each number. This is only a labelling difference and still represents the same age criteria.