I found this: http://www.ehow.com/about_5030458_legal-contract-age.html
Not sure how accurate it is but makes sense. I wonder if an appropriately nasty legal response could be brought against companies trying to engage underage minors into legal contracts.
This would have broad implications (good, imho)
Minor children are not of legal contract age, that is to say, with few exceptions, they cannot consent to or enter into a binding contract until they reach the age of majority.
To me, this means their clicked "consent" is non-binding. For any normal written contract, parents co-sign consent. Not so with click-through agreements.
Then comes my personal nitpick with clicked "consent". What's the legal proof of identity of the clicker? The crap filled in the "User Info" screen? Could be fake. So what's really bound to whom?