Microsoft did not make any mention of why that is, but as it turns out, it seems that there are some features of Cortana right now that makes it fall under the Children’s Online Privacy Protection Act. Basically what this means is that children under the age of 13 aren’t allowed to use Cortana or its features.
Given that Cortana also collects a ton of information from the user, especially considering that Microsoft is trying to make its voice assistant feature as feature-rich as possible, we can see why this might be an issue with lawmakers and parents who might not appreciate Microsoft collecting potentially sensitive information on their children.
However Cortana is in beta right now and perhaps Microsoft might release the final version with features that allows it to be used by children, but then again perhaps limiting Cortana’s features might not necessarily be the best way to showcase its capabilities. Either way hopefully we will learn more once Windows Phone 8.1 starts making its way onto the handset of end users.