Introduction of an "Age Content" meta tag for age restricted content
This isn't a be-all solution, but could address some of the issues with underage content for some of the more responsible websites.
You could create a standard meta tag that could be inserted at the top of each content delivering page. For example, Facebook sets meta tag at 14, or a BBC Iplayer program with an age set at 15.
This in itself wouldn't block content, but an agreed standard for meta age could then be incorporated into browsers/network/router layer to filter out age based on user defined settings.
Example meta tag could be;
meta name="contentage" content="14"
Search engines could be encouraged to include this tag within rankings to ensure a larger roll-out across sites. HTML5 is still in development so could be introduced as part of that with enough support/pressure.
Again - this won't stop illicit, or all sites but could be a step in the right direction for larger sites such as BBC, Google, Facebook, etc,etc.