The agency also demanded that Juul Labs turn over company documents about the marketing and research behind its products.
MORE: Doctor warns parents of dangers of Juul e-cigarettes
Those documents include reports on focus groups and toxicology, to determine whether Juul Labs is intentionally appealing to a particular market.
Nicknamed the iPhone of e-cigarettes, Juul devices resemble thumb drives and produce a small plume and smell like fruit or other flavorings.
Some say the smell is so concealable, students can even vape in class without it being noticed.
In a statement, Juul Labs said it agreed with the FDA, saying illegal sales of the company's product to minors is unacceptable.
The company says it is working with the FDA, lawmakers and parents to combat underage vaping.
Meanwhile, the FDA says it plans to issue similar letters to other manufacturers of popular vaping products.
MORE: Growing concern about 'Juuling' among teens in local schools