An early retiree quizzed over 140 millionaires about their money and found that most shared the same top regret: not investing even earlier

• John, who runs the personal-finance blog ESI Money, has spent the past few years interviewing millionaires.
• During his conversations, he discovered that many of the millionaires shared the same top regret when it comes to money: not saving or investing more in their 20s.
• John, a self-made millionaire who retired early at 52, said it is one of his biggest regrets as well.
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Hindsight is 20/20, even when you're a millionaire.

John, an early retiree who runs the personal-finance blog ESI Money, has interviewed more than 140 millionaires over the past few years and found that despite achieving the most coveted marker of financial success - a $1 million-plus net worth - many harbor regrets about the way they handled their money, career, or personal lives.

John wrote in a recent blog post that the top-cited money regret throughout his conversations had to do with timing: "Millionaires wish they had saved more money earlier in life."

"For many, not saving early (or saving enough early) can be an almost-fatal money move," John wrote. "It sacrifices years (or even decades) of compounding - something which can drive wealth at almost any income level. To give that up is a big, big regret."

Indeed, compound growth is one of the most powerful financial tools we have to build wealth. It may sound premature to squirrel away money for retirement in your 20s (or even earlier) - hey, it's decades away - but a few years could make a difference of tens of thousands of dollars, or more, thanks to compound interest.

How much could your investments grow?

John, a millionaire himself, reflected in a previous blog post about his own mistakes on the path to early retirement, saying he missed out on five years of saving and investing in his mid-20s.

He wrote: "If you take my net worth now and add five years of growth at even a conservative rate, it's probably in the neighborhood of $1 million - and that's just the compounding effect (no additional saving/investing). If I had saved and invested more it would be substantially larger. Oops."

Millionaire interviewee No. 40, a 35 year old with a $1.4 million net worth, regrets not starting early with a straightforward investment strategy: "I would have saved more as soon as I graduated from college and I would have invested entirely in index funds."

Some of the millionaire interviewees specifically regretted not utilizing an IRA, a type of tax-advantaged retirement account available to anyone, earlier in life. "I wish I could have maxed out my Roth IRAs as soon as I had a job in my early 20s," said millionaire No. 48, a 41 year old with a $1.6 million net worth.

"I would have started investing in an IRA as early as possible," said millionaire No. 49, a 51 year old with a nearly $2 million net worth. "Even though it was only $2,000 maximum contribution for the longest time, it still would have compounded and grown. I waited until my 40s to start contributing to tax-deferred investment accounts, which is STUPID."

Millionaire No. 70, a single mom with a $2.65 million net worth living in the Bay Area of California, said despite saving "diligently" in a 401(k) early in her career, she would frequently take out loans to pay for big expenses or bills.

"It was such a miss to not understand savings and compounding early on," she said. "Time is the most important thing in the context of money and I lost or blew 10 years of time and money. I can only imagine what good financial shape I'd be in if I had used those 10 years wisely."

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