You don't have to be a high school graduate — or an adult — to win over investors on ABC's "Shark Tank."
Take it from Nathaniel Wellen, a 15-year-old entrepreneur who appeared on Friday's episode of the show. The Los Angeles-based high schooler created The Duo, an umbrella with a second extendable handle designed for two people of different heights to stand beneath it comfortably.
Wellen came up with the idea at age 8, and teamed up with his father the following year to develop and design the umbrella, and obtain a utility patent for it, he said. He asked the show's investor judges for $100,000 in exchange for 10% of his company, which was pre-revenue and only had prototypes at the time of filming.
"That's a good size for a little compact umbrella," Lori Greiner said while testing a prototype. "You're a great example for kids out there who want to be entrepreneurs," Mark Cuban added.
The Duo came with a catch: Wellen already had another partner, weather accessories brand Shed Rain. "They own 49% of the company," he said. "They're going to help us [with] prototypes, manufacturing, distribution ... They're going to get us in stores like Target, Walmart, Kroger and Costco."
The judges were impressed that Wellen got his partnership by cold-calling the company and forging a relationship with the CEO. But Shed Rain was already doing what any investor could do, they said — with Cuban noting that Wellen might not want to take any deal the judges offered.
"If you dilute your equity at all, even 10%, you're under 50%. And that's an issue going into what you're doing," Cuban said. "Given the circumstances, you want to stay over 50%. for those reasons, I'm out, Nathaniel — but congratulations on what you've done."
Greiner, Barbara Corcoran and Kevin O'Leary also declined to make offers. O'Leary specifically cited Wellen's age as a concern.
"I'm very impressed with your presentation, but I have a basic rule: I have to be able to call my CEOs 24 hours a day," O'Leary said. "I don't like the fact that you're in school. What's your teacher going to say when I call your cell and you're in class? They're not going to let you answer, and that really pisses me off."
That made room for Robert Herjavec. "I love deals where there's a potential exit already built in," he said. "You create this product, you partner with them and it takes off ... There's no way they're not going to buy you out."
Herjavec offered $100,000 for 20% of the company. Wellen asked for 15%, and Herjavec countered with 18%. "You've got a deal, Robert," Wellen said.
On his way out, Wellen offered some advice for other kid entrepreneurs: "Just keep moving. Keep going. Reach for your goals and maybe you [too] will make a deal with Robert."
Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank."
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