Previously, women over the age of 17 could legally get Plan B and its generic counterparts over-the-counter, but younger girls needed a prescription. The history of getting this approval is a fascinating—and long—case of politics versus science.
The science at stake is actually pretty simple. Biologically, the medication works in the same way, and is safe, for women and girls of all ages. The important questions for younger girls included: Do girls under 17 understand how to use Plan B properly, without the help of a doctor? Do they know that it's not designed for use as a routine contraception? That it won't protect them against sexually transmitted diseases?