Why do you have to be 13 to register for social media sites?

I think all kids under 13 are missing out on all the fun. The sites they register for like Club Penguin, you can't do anything like update your status, makes videos or pictures etc. I don't think a 6, 7 or 8 year old should register, but maybe a 10 year old if they're mature enough. You can always fake an age, I know that, but still. The only one I don't think 13 year olds should use is MySpace because of all the viruses, spam, stalkers and inappropriate content. If 10 were called teen and 11 were called firsteen and 12 was called secondteen then the age limit would be a bit lower, maybe not.