Anyone selling a 12-certificate video game to a child under that age could face jail under a simpler and stronger rating system.
All games sold in the UK will be regulated under the Europe-widePEGI (Pan European Game Information) scheme, ending the role of the British Board of Film Classification (BBFC) in rating video games.
The BBFC had until now provided legally enforceable 15 and 18 certificates for games but it was never tasked with providing 12 certificates, meaning it was technically legal to sell a 12-rated game to younger children.
The new system makes all PEGI ratings made by the UK-based Video Standards Council (VSC)legally enforceable.
Culture Minister Ed Vaizey said: "The UK has one of the most dynamic and innovative video games industries in the world, and the games they produce not only entertain millions, but can also educate and foster creativity.
"Today's simplification of the ratings system benefits both industry and consumers and will help ensure that the millions of games sold in the UK each year are being played by the audiences they were intended for."
The PEGI system is specifically designed for video games and the age rating on the packaging will be accompanied by information about the type of content that led to it.
The VSC will have the power to refuse to grant an age-rating for a video game if it includes extreme content, meaning it would not be allowed to be sold in the UK.
Dr Jo Twist, chief executive of The Association of UK Interactive Entertainment (Ukie)said: "As we mark the start of PEGI as the single video game age rating system, we're delighted to use the opportunity to help parents to make informed decisions about which video games to choose for their family.
"We very much believe that the sole adoption of PEGI will provide clear and consistent direction on age ratings for parents and will be a vital tool in helping them to understand the types of games that their children should be playing."