Billionaire investor Warren Buffett told CNBC on Monday that if someone invested $10,000 in an index fund back in 1942, it would be worth $51 million today.

"The best single thing you could have done on March 11, 1942 — when I bought my first stock — was buy an index fund," Buffett told CNBC's "Squawk Box."

Since that time, Buffett said that Americans have seen 14 U.S. presidents, a world war, 9/11 and the Cuban Missile Crisis. "The only thing you had to believe in [1942] is that America would win the war and that America would progress as it has ever since 1776," he said. "The headlines were terrible every day."

Buffett, who was 11-years-old in 1942, said that people can be enticed into the stock market at the wrong time, adding all investors need to do is "buy a cross-section of America."