Even better would be if the website provided the age rating in a HTTP header, and the browser could locally check if the account is allowed to see it. That way you avoid exposing the age of the user.

And yes, even sending an age bracket exposes the age over time as you can observe a repeat visitor changing brackets and compute the actual age from that. With the server sending the info instead you can't really tell if the browser blocked it, or if the user just didn't navigate further on the page. (The browser still need to fetch all the CSS and other resources though, otherwise that would be possible to tell apart.)