Becoming the first billionaire may have been John D. Rockefeller’s destiny. By 1851, at the age of 12, he had saved $50 ($1,500 adjusted for inflation). Just 17 years later his company, Standard Oil, had become the largest oil refiner in the world. By September 1916, Rockefeller’s net worth was beyond a once unthinkable number: $1 billion ($30 billion adjusted for inflation).