Why people under the age of 13 are not allowed to use this site ?
Because generally it is believed that younger members are more easily tricked into revealing their original identity and submitting private details about themselves (especially if there is no supervision involved), which can lead to serious problems to the unsuspecting child later on..
There's privacy laws in the U.S. that prevents companies from collecting information from children under 13. Since you have to actually put in your information to be able to use the features on this site, they can't allow under 13 year olds to use the site
Because it's illegal to take personal information from anyone under 13 on the internet.