That's a thorny question. The main ways we currently got either involve the sites in question collecting personally identifiable information, such as government issued ID, and making a determination as to what to serve based on the information that contains, or the sites adhereing to a voluntary code, such as RTA, to include an identifying header, and parents installing, configuring and maintaining the software or services to restrict access. The former method is obviously dangerous as it requires handing over your ID, and the latter is all voluntary and so there is little impetus to do it, and the complexity will also act as a deterent for parents. Turning it around and just having the computer send a flag for the age bracket gets rid of the need to transmit personally identifiable information and makes the parents' setup job much easier with a one-shot, tick a box and carry on, process.