The investing advice Jack Bogle gave Tony Robbins for 20-somethings is useful for just about anyone

Jack Bogle, retired founder and CEO of The Vanguard Group, created the very first index fund in 1976.

Since then, the index fund - a type of mutual fund pegged to specific market index, like the S&P 500 - has been lauded as an investing tool by legendary investors such as Warren Buffett, who called Bogle a "hero" in his 2017 letter to Berkshire Hathaway shareholders.

Bogle is also one of the experts performance coach Tony Robbins interviewed for his latest books: "MONEY: Master the Game" and "Unshakeable."

In a conversation hosted by Money, Robbins and Bogle talked about investing, 401(k)s, and how fees can erode your returns.

Then, they started discussing 20-somethings. "Jack, what do you tell people right now who are on the sidelines? Especially younger millennials," Robbins asked.

"Well, I can tell you what I say to them, but you'll have to figure out what they actually do," Bogle answered.

Bogle continued:

"If you're not investing, now is the time to start. The best single thing that can happen to [young] investors who are accumulating money is a major and sustained market decline. Think about that. And that's not the best thing that can happen to somebody who already has their capital in.

"There aren't any easy answers. We will eventually get to the place where the consumer - the investor consumer - understands what Tony and I are talking about here today and behaves accordingly. Regular investing, diversified investing, and above all, low-cost investing."

In a Facebook Live Q&A with Business Insider in February, Robbins shared some of the thinking behind Bogle's emphasis on getting into the market.

When asked about the best investing advice for the average person yielded by the 50 interviews with top investors he conducted for his books, Robbins said that Bogle told him, "Tony, the secret is do nothing. Just stand there."

He's recommending you leave your investments alone.

Many people think they have to be investing professionals to put money in the stock market, or that they should be trying to beat it - buying and selling regularly based on market fluctuations to try and avoid losing any money. "People don't invest because they're afraid of losing," Robbins said in the Facebook Live. "They're afraid of losing because they see corrections or crashes or imagine them coming."

But in the long term, Business Insider's Richard Feloni reported, "the trend for the last 200 years is that the overall market continues to grow over the long term thanks to inflation, productivity, and population growth."

As Business Insider previously reported, Warren Buffett told Bogle in "The Little Book of Common Sense Investing" that "a low-cost index fund is the most sensible equity investment for the great majority of investors. By periodically investing in an index fund, the know-nothing investor can actually out-perform most investment professionals."

During the conversation with Money, Bogle repeated a message he wrote in the forward to "Unshakeable": "We live in an uncertain world and face not only the risks of the known unknowns but also the unknown unknowns - the ones that we don't know we don't know. Despite these risks, if we are to have any chance for meeting our long-term financial goals, invest we must."