The charges follow a 10-month investigation under the bloc's Digital Services Act, which requires large online platforms to do more to tackle illegal and harmful content.
"Children are accessing adult content at increasingly younger ages and these platforms must put in place robust, privacy-preserving and effective measures to keep minors off their services," EU tech chief Henna Virkkunen said in a statement on Thursday.
The European Commission, charged with enforcing the act, said the companies did not use objective and thorough methodologies to assess the risks to children accessing their services.
It accused Pornhub, owned by Cypriot group Aylo Freesites; Stripchat, a subsidiary of Cyprus's Technius; XNXX, owned by Czech group NKL Associates; and WebGroup Czech Republic unit XVideos of being more worried about their reputations than societal risks to minors.
The regulator also took issue with the companies' self-declaration tool that allows users to access their platforms with a simple click confirming they are over 18.
It said this and measures such as page blurring and content warnings were not effective to stop children accessing their sites.
The commission said Pornhub, Stripchat, XNXX and XVideos needed to implement privacy preserving age-verification measures to protect children from harmful content.
Companies risk fines as much as six per cent of their global annual turnover if found guilty of Digital Services Act breaches.
