Tool to protect children's online privacy. A new study of 100 mobile apps for kids found that 72 violated a federal law aimed at protecting children's online privacy. Researchers developed a tool that can determine whether an Android game or other mobile app complies with the federal Children's Online Privacy Protection Act (COPPA). Source 1f.
A lot of sites do their part by asking for this during registration but I'm not sure about apps. I think it is really hard to enforce this if children can lie and say they are older than 13.