Also the name of a
popular American-based
girls
magazine.
Seventeen features articles on fashion, beauty, celebrities, love, sex, and other things that would be of interest to teen girls.
Issues are monthly (so, 12 a year).
Seventeen is based in New York and is a part of PRIMEDIA, a company which publishes hundreds of magazines.
As for why a 20-something male like me knows anything about Seventeen, I helped them with public relations campaigns at a previous job.