The developers of the Juul e-cigarette say they created the device to help adult smokers quit. But their product achieved enormous success, becoming the top-selling e-cigarette in the US in two years, in part by attracting a huge following among kids younger than 18, who aren’t legally allowed to purchase such products. Now, the nation’s Food and Drug Administration has banned Juul Labs Inc.’s products from the market, noting their “disproportionate role in the rise in youth vaping.”