That was in 2005, right before Adeney's 31st birthday.
They didn't win the lottery or inherit a fortune - they weren't even earning abnormally large paychecks. Combined, their salary amounted to $134,000 a year.
They did it "simply by living a lifestyle about 50% less expensive than most of our peers and investing the surplus in very boring, conservative Vanguard index funds and a rental house or two," Adeney writes on his blog.
Today, 11 years after retiring, he's earning about $400,000 a year through his blog, but he and his family spend just $24,000 of that for annual expenses.
Despite having "never heard of personal finance," as he told Nick Paumgarten of The New Yorker in a recent article, he's clearly figured out how to master his money, a skill that he's passing down to his 10-year-old son, Simon.
One simple strategy involves a bike. Adeney pays Simon 10 cents for every mile he logs on his bike, Paumgarten reports. He's up to 1,300 miles - which is $130 of biking money - and has $700 of total savings. Not bad for a 10 year old.
What's more, Simon's savings are sitting in the "Bank of Mr. Money Mustache" and growing at an annual interest rate of 10%.
In addition to passing along a healthy lifestyle - Adeney also bikes year-round, going through just two and a half tanks of gas a year - he's teaching his son the power of compound interest, which, if leveraged at a young age, can get you to the seven-figure mark by retirement.
While retirement is a bit far off for Simon to concern himself with, college is less than a decade away. With regard to that expense, Adeney told Paumgarten, "I would rather Simon find a way to pay for it himself."