From the Mad.com terms of use:
"You affirm that you are either more than 18 years of age, or an emancipated minor, or possess legal parental or guardian consent, and are fully able and competent to enter into the terms, conditions, obligations, affirmations, representations, and warranties set forth in these Terms of Service, and to abide by and comply with these Terms of Service. In any case, you affirm that you are over the age of 13, as the MAD Website is not intended for children under 13. If you are under 13 years of age, then please leave MAD immediately - there are lots of other, more pleasant web sites out there that won't horrify your delicate parents. Talk to your parents about what sites those might be. "
So the website owners make it clear that children under 13 shouldn't be on their site. Beyond that age this becomes relevant:
http://www.bbc.co.uk/news/education-26049333