I remember that this was trying to be a thing in the mid-2000s (via COPPA) in order to help better age restrict adult content. Sites would use metadata to signa

I remember that this was trying to be a thing in the mid-2000s (via COPPA) in order to help better age restrict adult content. Sites would use metadata to signal the apropriate age limit for their content and web filters could use this to deny access. I don't think that ever went anywhere.