My kid (under 13 y.o.) and all her friends know that when you sign up for some online service, you always need to give a birthday such that your age is 13+. Many services are totally nerfed to the point of uselessness if you say you are under 13, and/or won't even let you sign up. Also, some companies blacklist your E-mail address if you ever say you are under 13, so you can't try to sign up as a 10 year old, realize your mistake, and then try to re-sign up with the same E-mail as a "14 year old". Consequently, circumvention techniques get around pretty quickly among the pre-teen circles.