A self-made millionaire says a decision he made at 21 was not only the key to building his wealth, but keeping it intact

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A self-made millionaire credits his wealth to investing in retirement accounts from an early age. (The millionaire is not pictured.)

• A self-made millionaire featured on the personal-finance blog ESI Money credits his wealth to investing in retirement accounts from an early age.
• The man, who shares a $1.1 million net worth with his wife, has been saving about 15% of his gross income in retirement accounts, including a 401(k) and Roth IRA, for more than two decades.
• He said investing in tax-advantaged retirement accounts that penalize early withdrawals has been the key to keeping his savings intact.
• Visit Business Insider's homepage for more stories.

Imagine if you could thank your 21-year-old self for making you rich.

Such is the case for the man featured in the latest millionaire interview on personal-finance blog ESI Money. John, who runs the blog and does not share his last name online, talks to a 47-year-old worth about $1.1 million who credits his financial success to a decision he made more than two decades ago.

"I can say that I would not be a millionaire today if I hadn't started to invest 26 years ago. Most of my net worth is investment gains," said the man, who earns $185,000 a year as a director of production at a manufacturing company.

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"Investing in my retirement plans has been the key to my wealth," the man told John. His 401(k) is his largest asset at nearly $449,000. He also has a separate profit-sharing plan worth about $336,600 and a Roth IRA worth over $106,300. Additionally, his wife, who works at a hospital, has about $40,400 in a 403(b) plan, $17,500 in a Roth IRA, and nearly $13,000 in an IRA. The couple also has a brokerage account with about $34,900, plus about $48,000 in cash, and home equity of about $75,600.

The man told John that 15% of his gross income has been going into his retirement accounts "pretty much my whole working life," while his wife has been saving the same portion of her $30,000 income for about eight years.

"I work with people who say they can't afford to save money in their 401(k). I say you can't afford not to save. Compound earnings over time are so powerful and are how I've accumulated what I have so far," the man said.

But it's not just starting early and having the discipline to save that helped him build a seven-figure net worth. Keeping the majority of his savings in tax-deferred retirement plans that enforce penalties for early withdrawals, rather than cash or regular taxable investment accounts, has acted as a safety valve.

"I am a good saver, but it is too easy to spend your savings," the man said. "Retirement funds are protected from spending and are allowed to grow."

Financial expert Ramit Sethi also believes in the power of investing early. In the second edition of his bestselling book " I Will Teach You To Be Rich," Sethi writes that "investing is the single most effective way to get rich."

Sethi said too many people are sitting on the sidelines of the stock market, all but ensuring their money won't multiply, but they probably don't realize they need no expert-level knowledge of stocks to invest. Sethi says most people can, and should, start with automatic contributions to retirement accounts.

"You're worried about maybe losing money, you are losing money - and that's the great irony," Sethi told Business Insider. "The worst mistake you could make is to wait, is to say somebody is going to come save me. It's not going to happen. And every day you wait, you're actually losing."

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