Linux

First off, we're talking about operating systems and not websites.

Second, if it's open source, the users can just edit out the age verification, no matter how it's implemented. And if it's open source, you simply can't prevent that behavior.

In theory, the closed source systems make that more complicated, to the point where we can "rely" on those safeguards.

There's a logic here, even if I totally disagree with the premise of the OS needing anything to do with the user's age. Parents need to parent and we've had solid enough parental controls for a long time now that make this type of monitoring and control easy.