Sorry to keep nit-picking, but even the term minor has a variable age range, even again in the USA.
The states of Georgia, Illinois, Louisiana, Massachusetts, Michigan, Missouri, South Carolina, and Texas define a minor as a person of under 17, Connecticut, New York, and North Carolina define a minor as someone under 16.

This is, however, completely irrelevant on 2 grounds.

The first being that the age of a person legally being able to sign a document (release form) is based on the age of consent and not the age of majority.

Secondly none of my Poser characters seem to have learned to write yet! ;)