They have a 'parental consent form' in their FAQ http://diamondheartpress.com/submission ... sent-form/
On one hand they are a new publisher, on the other, they are listed as a pro publisher in Duotrope https://duotrope.com/market_7141.aspx who pays pro advances, which is super rare. Unheard of, for first time teen writers, if you ask me, Paolini accepted.
Anyway - all you guys who are not adults yet but wanna have a go at a pro market - here it is. This publisher will either sink or swim, and if enough quality teen fic is submitted to them - they won't sink. Maybe.
