Consider this for a moment.... My child gets online and goes to google to do some research for a school project. Google senses that the user is a child and does not display any ads (OK, maybe I'm dreaming) or displays child appropriate ads. This kind of claim "under 13" could be tied to an anonymous identifier so that each time my child interacts with a web site, they aren't being tracked and all their actions correlated to build a profile about them (of course there are many ways to do the correlation; using an anonymous identifier just makes it a little harder). The next piece of the puzzle would be for user agents to expose this information directly to the web site through some means.
My one concern is whether "pushing" the claim of "under 13" leaks too much information. Would this enable someone to target my child because they know s/he is "under 13"? However, I fear that if the claim is not "pushed" most web sites would just ignore it and not bother to go check with the appropriate attribute provider. Maybe the only way to solve this is through legislation (any one want to sponsor a "No ads for kids" bill?). Until then Firefox2 + AdBlock Plus is your friend.
I'd be interested to hear if anyone else has ideas or even considers this a significant problem.