Kenya Forrest, Jr., 20, says sneaker trading got him interested in learning to trade stock options. Source: Kenya Forrest, Jr.

Formal, part-time jobs for kids are so last century — the percentage of teens with jobs peaked 40 years ago — and it's all about side hustles now. Young people may go into business because they want some spending money, and some high-profile teen entrepreneurs have made bank to the tune of hundreds of thousands of dollars selling, say, sneakers. But for every Aaron Maresky or Benjamin Kapelushnik, there's someone who is gaining something even more valuable. A side hustle gives a teen so much more than just money, says Thomas Henske, a certified financial planner with Lenox Advisors in New York. "It's not until you actually have to earn the money that you appreciate what it can buy and what things cost," Henske said. Here's what these entrepreneurs learned from earning cash.

College-level courses

Lessons learned from a successful sneaker side hustle: consistency, the value of a dollar and, says Kenya Forrest, Jr., age 20, stock options trading. For several years in high school, Forrest worked at part-time jobs to save up for a pair of Air Jordan reissues. He was lucky and got a valuable pair at a great price. It was, he says, the shoe he had dreamed about since the 9th grade. Forrest, now in college, then learned to keep track of expenses as a sneaker reseller — "It's really important to do your bookkeeping," he said — and calculate the value of starting his own brand, which would allow him to command higher prices.

His successes have piled up. He recently got his license as a real estate broker. He had $20,000 in sales in one year in college, and $80,000 or so last year. He sailed through college economics courses. "I was taking microeconomics and I got an A," Forrest said. "The whole time, I was just applying everything I do on a daily basis." When his instructor taught concepts such as pricing and scarcity, what drives people to buy certain things or why prices are sometimes higher, Forrest translated this to shoe terms: "The branding of a certain shoe, and stock levels," he said. If a manufacturer produces 60,000 pairs of a certain model, and you know there are millions of people who want it, it explains how marketing comes into pricing. "Reselling has opened my eyes to gaming as many income streams as possible," Forrest said. He is also looking to explore investing in stock options.

Marketing 101

Adam Houser, now 38, has a part-time restaurant job in Boulder, Colorado. "Everything else is reselling, as I'm working toward becoming a photographer," he said. But his first side hustle was selling candy between classes in middle school at age 13. He learned something about competition — "A fellow student was doing it as well," he said — and the value of buying at wholesale for a retail market. Houser realized he could provide a better service at a better cost. The other kid had dollar-store brands and his business was random, selling to friends once in a while. Houser's mother introduced him to someone who could sell him candy at better pricing, provided he bought in bulk. More from Invest in You:

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Dream of being your own boss? Take this quiz A Blow Pop that cost him 20 cents could be sold for 50 cents. He repackaged his other big seller, Warheads, in individual plastic bags and sold everything in increments of 25 cents, 50 cents or a dollar. "I kept it simple," he said. "Everyone had quarters." He explained pricing to other kids who thought his prices were high, though most did not: "I would sometimes say, 'It's not like you can take 20 minutes and go to the mall and get it yourself," Houser said. Sometimes he'd sell at cost to a new customer, but explain that the Blow Pop was a quarter for the first-time sale only.

Understanding true cost

One of the biggest lessons, though, was the importance of unit pricing. "To this day, at the grocery store, I look at the unit pricing on a box of cereal to understand the true cost," Houser said. Houser credits selling candy with his passion for reselling and entrepreneurism. He also learned the value of money. "I would see some of my friends who couldn't buy this or that, and I had extra pocket change" to spend, he said.

Time management

Creating your own business means more risk and more headaches. Kids learn there's a balancing act between doing the actual business — mowing lawns, for instance — and sales and marketing. "They need to try to drum up business and try to market it," Henske said. They can learn billing and receivables: They might be mowing a lawn for someone who's out of town, and they'll have to figure out how to get paid. "They may need to think about how to scale it," Henske said. If there's more business than one kid can handle on their own, they have to consider hiring other kids and paying them a salary.

'We give back'

Shawn Dadon, 21, at left, and his brother David Dadon, 19, are sneaker designers and philanthropists. Source: Shawn Dadon

Shawn Dadon, 21, and his brother David Dadon, 19, have been flipping and selling sneakers for about six years. They also design and do custom work on shoes. But some who sport a pair of Jordans from the brothers didn't pay a cent. "We do it for the love of the game," Shawn said. When the brothers can help a younger kid who can't afford high-end sneakers, they take pleasure in giving away Champion sportswear and Nikes. They look in particular for kids who are being bullied or who have special needs. Some years back, the brothers could not afford their own sneakers. A friendly neighbor gave David a pair of Jordan Laker 6s, a reissue that later skyrocketed in value to over $500. He cherished them and kept them in immaculate condition. But it always ate at him that his brother didn't have a great pair of kicks. Finally, David sold the Nikes, making enough to buy matching sneakers for himself and Shawn.