Mark Zuckerberg — Billionaire at 23

Zuckerberg became a billionaire in 2008 at age 23 — three years after dropping out of Harvard to build Facebook. By 25, he was worth $6.9 billion. By 40, he's worth $180 billion with 3.07 billion monthly active users across Meta's platforms. He turned down Yahoo's $1 billion acquisition offer at 22 — a decision that looks like the greatest "no" in business history given Meta's current $1.5 trillion market cap. The dorm-room coder who became the youngest self-made billionaire in history wrote the playbook every founder on this list tried to follow.