A man who became a millionaire in 10 years shares his best advice for 20-somethings who want to do the same

In 2004, Justin landed his first job out of law school, working at an engineering consulting firm for $48,000 a year. Frugal by nature, he and his wife began saving diligently and maxing out their retirement accounts each year.

By 2013, the couple had saved up over $1 million - enough for Justin to retire at 33.

Justin and his wife - who share their story on his blog, Root of Good - never won the lottery or played the stock market. Justin's salary topped out at $69,000, and wife's only reached as high as $74,000. But he believes anyone can reach financial independence before they reach traditional retirement age.

To 20-somethings hoping to follow in his footsteps, Justin's advice is straightforward: Save early and save often, as he told the Mad Fientist on an episode of his "Financial Independence Podcast."

When you're still young, how you save doesn't matter as long as you're actively putting away money in one way or another, Justin underscores.

"Figure out how to invest it as you go along," he told the Mad Fientist. "The planning aspects of it, how much you need to save, your budget, your withdrawal rates, and all that - that's a lot easier to figure out later in the game."

Time is one advantage you can never get back if you're planning to retire early, so don't worry about knowing all of the ins and outs of taxes and investments from the start. "Start saving today, and then get smarter tax planning as you learn more, as you get along," Justin said.

To those later in the game, but still on the path to early retirement, he says to "just stick with it."

Perseverance is half the battle in achieving any financial goal.

Justin explains: "Stick with those plans and do what you want to do for yourself because you want to do them and not because you're buckling into the peer pressure of, 'Well, everyone else is buying a Mercedes. ... Everyone else is going on these crazy, expensive vacations to resorts in Tahiti.'"

"Persevere until you get to your end goal," he advises.