Of course there is the issue of clearing into other countries. Places that she would stop might have an issue with such a young person being un-attended. Then what about the unscrupulus out there (pirates notwithstanding)?
I am not so sure about that. If she has a proper license (a non-professional captain license) issued by the authority of her country of origin, she can probably enter and leave any port.
In Brazil, for instance, one doesn't need an official license to sail small recreational boats without an engine up to certain limits.
At sixteen one is allowed to apply for the first official license, good for recreational boats in an enclosed body of water.
Two years after receiving the first license (meaning more than 18), one can apply for a recreational coastal permit.
Another two years after receiving the second license (meaning at least 20, but usually 21) one can apply for a recreational worldwide permit, boats up to 20 tons.
This means that it is impossible for a Brazilian to obtain the license required to sail around the world before he is 20 years old. However, a foreigner with an equivalent worldwide permit should be allowed to stop and leave, regardless of age. The foreign boat is foreign territory and the laws and regulations regarding its captain's license are those of the foreign country.
