SmileDirectClub’s IPO Creates a Pair of 30-Year-Old Billionaires

Even after one of the worst stock-market debuts in more than a decade, two 30-year-old founders of SmileDirectClub Inc. still have plenty to smile about.

Alexander Fenkell and Jordan Katzman are both billionaires, as is Katzman’s father, Chief Executive Officer David Katzman, 59, following an initial public offering of shares in the maker of dental aligners.

SmileDirectClub plunged 28% to $16.67 Thursday, making it the first U.S. firm since at least 2008 to raise more than $1 billion and price its IPO above range, yet fall in its inaugural trading day. The shares, offered at $23, rebounded Friday, climbing ...