Looks like Valve's rolled out a new age verification process for Australian Steam users. To access R18+ games, you now need to verify your age using a credit card. This has been reported by several users, so it seems to be legit.
Not sure how that's gonna sit with everyone down under.
I get the reasoning behind age verification, but tying it to credit cards feels a bit excessive. It could alienate younger players or those who aren't comfortable sharing that info, which is a bummer. There has to be a better way to manage access without potentially pushing away a segment of the audience looking to enjoy R18+ titles.