4. Evan Spiegel, 27 — $4.1 billion (£3 billion).
Spiegel cofounded the popular photo-sharing app Snapchat with his former Stanford classmate and fraternity brother Bobby Murphy in 2011.
When Snap began trading in March 2017, he became the youngest public company CEO.
Spiegel and Bobby Murphy each own around 18% of Snap but have voting shares that give them control over the board.
Some 170 million people use Snapchat each day, according to Forbes.
1. Alexandra Andresen, 21 — $1.4 billion (£1 billion).
Alexandra Andresen is the world's youngest billionaire at the age of 21.
She inherited 42% of the family-owned investment company Ferd along with her sister Katharina.
Ferd is an active investor on the Nordic stock exchange. It also has private equity investments and runs hedge funds.