While Cortana has been getting all sorts of attention since its debut, not everyone is happy with Microsoft’s new digital assistant. The US government apparently falls within that category, as the service apparently violates the Children’s Online Privacy Protection Act (COPPA).
This means that kids under the age of 13 are restricted from using it. Microsoft seems to have considered this and if it detects the user could be underage then it returns “I'm sorry, you'll need to be a bit older before I can help you”. This is because Cortana, by its very nature, collects personal data and sends it to Microsoft for processing in an effort help you and customize itself to the user.
The voice assistant is still in beta, and perhaps there will be changes made, but it is unlikely that this will be one of them, as it would break the functionality of Cortana.