Final answer:
Bill Gates became a billionaire at 31 years old when Microsoft's stock increased significantly after the company went public.
Step-by-step explanation:
Bill Gates became a billionaire at the age of 31. He achieved this financial milestone in the year 1987, a time when Microsoft, the company he co-founded, went public and experienced skyrocketing stock values.
Gates's contribution to the proliferation of personal computers with his company's Windows operating system was instrumental in building his fortune and establishing Microsoft as a dominant force in the technology sector.