The women's group that sued over the age limits said Tuesday's action is not enough, and it will continue the court fight if necessary.
Lowering the age limit "may reduce delays for some young women but it does nothing to address the significant barriers that far too many women of all ages will still find if they arrive at the drugstore without identification," said Nancy Northup, president of the Center for Reproductive Rights.
The FDA said the Plan B One-Step will be packaged with a product code that prompts the cashier to verify a customer's age. Anyone who can't provide such proof as a driver's license, birth certificate or passport wouldn't be allowed to complete the purchase. In most states, driver's licenses, the most common form of identification, are issued at age 16.
"These are daunting and sometimes insurmountable hoops women are forced to jump through in time-sensitive circumstances, and we will continue our battle in court to remove these arbitrary restrictions on emergency contraception for all women," Northup said.
Other contraceptive contraception advocates called the move promising.
"This decision is a step in the right direction for increased access to a product that is a safe and effective method of preventing unintended pregnancies," said Sen. Patty Murray (D) of Washington. "It's also a decision that moves us closer to these critical availability decisions being based on science, not politics."
Social conservatives had opposed any efforts to loosen restrictions on sale of the morning-after pill, arguing that it was important for parents and medical professionals to be involved in such decisions involving young girls.
Half the nation's pregnancies every year are unintended, and doctors' groups say more access to morning-after pills could cut those numbers. The pills contain higher doses of regular contraceptives, and if taken within 72 hours of unprotected sex, can cut the chances of pregnancy by up to 89 percent. But it works best if taken in the first 24 hours.