As teenagers in the Netherlands, Jobert Abma and Michiel Prins hacked into the high school TV station. When they were in college, they told an education software company that its platform leaked grades and personal data. In 2012, Alex Rice, then Facebook’s head of product security, challenged them to find a vulnerability. They did, scored a contract with Facebook, and started hacking into Twitter, Spotify, and Uber to win work from them, too. Along with Merijn Terheggen and Rice, they started HackerOne in 2012, to run “bug bounty” programs--which reward hackers for finding security flaws--for Starbucks, GM, Uber, the U.S. Department of Defense, and about 1,000 other organizations. Such customers pay subscriptions, costing from the low four figures to tens of thousands per year. --Will Yakowicz

When Siqi Mou was in college, she suffered from acne and other skin conditions. Her friends recommended over-the-counter products they liked--but none worked. "Why is there no personalized solution?" Mou remembers thinking. Later, when she was a news anchor in Indonesia, a skin care company asked her to star in an ad. She turned it down after learning the products were loaded with harsh chemicals. Her frustrations led to HelloAva, which creates personalized skin care regimens. Users answer questions about their skin via chatbot, attach a selfie, and receive recommendations. (They can also consult with an aesthetician.) The company takes a cut of online sales made through its platform. "If I had never gone through this experience" of having bad skin, says Mou, a concert pianist who once performed on the Great Wall of China, "I would have never figured this out." --Emily Canal

Alexander Harmsen has been interested in aviation since he was a teenager. "I got my pilot's license before I even got my driver's license," says Harmsen, who was 17 when he started flight training. Now, along with his college friend James Howard, he wants to help drones pilot themselves, without humans having to dictate every move. Their company, Iris Automation, uses the same technology that's propelling driverless cars--and they think it'll be revolutionary for a variety of industries. 

One current use-case is inspecting railway lines for targeted maintenance. Harmsen says that the company has roughly two dozen clients in five countries. Due to strict regulations, "every single drone that's flown right now has to have a pilot and a visual observer with their hands on some sort of remote control," which limits the drone's flight to about a mile. That changes when you have a full-fledged "sense and avoid" system like the one Iris Automation is building. --Sonya Mann

In the spring of 2010, Alex Riley was working a construction project in Detroit and discovered that not all laborers are suited for the same tasks. “We had a labor force of 150 demolition employees,” he recalls, “but we were supposed to be doing a residential rehabilitation--carpentry, plumbing, roofing, masonry.” After a frustrating experience trying to recruit workers, Riley--along with co-founders Patrick Beal and Paul Kaser—built a staffing website, billing it as “LinkedIn for contractors.” Now, besides connecting talent to local employers, MeritHall and its affiliate companies find road salt for clients, offer facility management and vocational training services, and provide management consulting--all the while helping to revitalize the Motor City. “We really focused on getting Detroiters to work Detroit jobs,” Riley says. --Zoë Henry

Almost 10 years ago, Suhail Doshi founded Mixpanel, a near-unicorn analytics startup, when he was practically fresh out of college. Doshi learned how to navigate the professional world straight from the CEO position, alongside co-founder Tim Trefren (who only recently turned 30). Before too long, Mixpanel quickly reached $40 million in revenue, without "doing a lot of explicit sales," according to Doshi. "There were a lot of growing pains," Doshi recalled. "But we also had a little bit of luck. And the luck was that we ended up building a product that people really, really loved." Doshi hit on the right idea at the right time, and he managed to convince the boss of a company where he'd interned to invest. That boss happened to be Max Levchin, one of PayPal's legendary co-founders. With Levchin's help and a stint in Y Combinator, Doshi and Trefren set out to revolutionize the business world's data expectations. Now you couldn't just measure page views and call it good--Mixpanel and its peers opened up a world of user engagement data that changed how software products were created and judged. It's a far different set of challenges from what he'd envisioned: Once, Doshi's goal was a $90,000 salary as an Intel engineer. --Sonya Mann