What is it possible to accomplish if you put your mind to it? Sometimes there's no better place to look for inspiration (and a healthy push to expand your dreams) than those who managed to achieve incredible things at a (relatively) early age.

Each year the World Economic Forum puts together a list of the most impressive scientists, academics, activists, and entrepreneurs under 40 from around the world. I trawled through the 121 names in the latest edition to find the American founders who made the cut.

From turning radioactive waste into usable energy to regrowing human bones, these entrepreneurs have already achieved amazing things and they haven't even hit their fifth decade yet. The list is a powerful testament to what founders can accomplish if they dream big... really big.

1. Sam Altman, President, Y Combinator

What has Altman accomplished before the age of 40? Oh, just heading "one of the world's most successful startup accelerators who has supported well-known names like Airbnb, Dropbox and Zenefits. He is known for helping other entrepreneurs succeed by funding and mentoring startups in order to solve the world's problems."

2. Leslie Dewan, CEO, Transatomic Power Corporation

"An exceptional entrepreneur developing safe and environmentally-friendly nuclear power plants and on the brink of launching a technology that reuses radioactive nuclear waste." What are you accomplishing at work?

3. Aria Finger, CEO, DoSomething.Org

Finger "runs a non-profit that helps millions of young people to take action and participate in issues in their community. She has tripled corporate funding for her organisation, quadrupled the number of teens who take part in campaigns and launched Teens for Jeans, which has clothed 5 million homeless teenagers."

4. Joe Gebbia, Co-founder, Airbnb

Gebbia "co-founded and led Airbnb, one of the world's most successful start-ups. He is also recognized for launching innovative programs to leverage Airbnb's platform to find housing for victims of natural disasters."

5. Roy Hessel, CEO, Clearly

"The founder of three organizations linked to opthalmic eye care, all focused on using technology to make prescription eyeglasses and other forms of vision correction affordable and accessible to everyone worldwide," Hessel clearly has a pretty impressive track record.

6. Vanessa Kerry, Co-founder, Seed Global Health

Besides being the daughter of the Secretary of State and a critical care physician, Kerry co-founded an organization that "is working to close the gap in qualified healthcare professionals in underserved markets and developing countries through training and education of local nationals to build a pool of capable local talent."

7. Ashton Kutcher, Co-founder, THORN: Digital Defenders of Children

You might know Kutcher as that guy from That 70s Show, the World Economic Forum describes him as a "tech investor who set up an innovation lab to address the sexual exploitation of children and the proliferation of child pornography."

8. Avid Larizadeh-Duggan, Partner, Google Ventures

Larizadeh-Duggan is "one of the leading figures in the venture capital world as a general partner with Google Ventures."

9. Michael Lefenfeld, President and CEO, SiGNa Chemistry INC.

"He has developed green chemistry materials that are rapidly being applied in industries for safer products, better medical technologies and revolutionize the development of new alternative energy solutions," explains Lefenfeld's bio.

10. Andy Moon, CEO, SunFarmer

Moon is "a solar energy entrepreneur and founder of a social enterprise that builds, finances, and operates solar energy projects in developing countries."

11. Danae Ringelmann, Founder, Indiegogo

Ringelmann is the "founder of a pioneering crowdfunding platform and credited with paving the way for the democratization of access to capital across the globe."

12. Shivani Siroya, Founder, InVenture

Siroya "founded a data-led microfinance company that helps people access credit through phone apps with an aim to unlock credit for 2.5 billion people in the informal economy."

13. Nina Tandon, President and CEO, EpiBone

Tandon "founded the world's first company growing living human bones for skeletal reconstruction."

14. Julie Yoo, Co-founder, Kyruus

"Her company's technology uses data science to make health providers and hospitals smarter about matching patients with physicians," says the World Economic Forum of Yoo.

15. Monica Yunus, Co-founder, Sing for Hope

Yunus has a unique combination of professional accomplishments. She's an "opera singer and social entrepreneur, who has co-founded a non-profit, dedicated to positive social change by bringing arts outreach programmes to communities in need and pilots initiatives."

16. Yao Zhang, Founder, Roboterra

Zhang is "a renowned education entrepreneur with two successful startups under her belt," according to her bio. "Yao has introduced robotics within education, through the use of colorful robotics learning kits designed for beginning. She developed a leading curriculum solution company, serving more than 80 universities."