"Ofcom’s guidance on the OSA states that one highly effective age assurance measure is credit card checks," Valve says. "This is because, in the UK, an individual must be at least 18 years of age to ...

Valve has introduced Steam age verification for UK users, meaning you'll have to supply valid credit card details to access mature pages.

Folks in the United Kingdom wanting to shop for mature games will have to make sure they have a valid credit card on file ...

Valve have a Steam blog up, which explains the process for age verification and the need for a credit card in particular.