The “4 percent rule” is a bedrock of retirement planning. But does it apply to those who quit working before 65? The rule of thumb holds that retirees who spend only 4 percent of their investment portfolio annually, adjusted for inflation, will be able to stretch out their savings for the rest of their life. For example, a $1 million brokerage account gets you $40,000 a year to spend.

Lately, the 4 percent rule has been under assault, with experts warning that the future could bring weaker market returns, an increased life span, or both. “If you retire at 40 with a couple million dollars, you’re going to worry—about financial emergencies, taxes, inflation, market crashes, and the chance you’ll live a lot longer than you’d planned for,” says Robert Karn, an adviser with Karn Couzens & Associates in Farmington, Conn.

Evan Inglis, an actuary at Nuveen Asset Management, offers an alternative rule: Divide your age by 20—couples should use the younger partner’s age—to get the percentage that you can safely spend. For a 40-year-old, that’s 2 percent, or $20,000 a year on $1 million in savings.

How do these concepts play out in the real world? We asked three people who retired in their 30s and 40s to explain how they’ve made it pay off.

The Stay-at-Home Dad
Joe Udo retired in 2012, at 38, after spending 16 years as a computer hardware engineer at Intel and saving aggressively. He and his wife, a human resources professional, performed a two-year test run in which they supported themselves on her salary alone while meticulously tracking their spending.

Financial planners typically recommend that you save enough to replace 80 percent of your preretirement income, but Udo says his family lives comfortably on less. They spend about $50,000 a year—from his wife’s earnings, dividends from stocks, rental income, and what Udo makes from a blog where he chronicles his retirement experience. “Right now we have plenty of padding in our lifestyle,” he says. “Our income is more than our expenses.” They have about $1.4 million in a brokerage account and rental properties worth $600,000.

Udo’s wife plans to retire by 2020, if not sooner. In the meantime, the couple continue to sock away money, including $400 a month in a 529 college savings plan for their son. Udo’s plan is to keep a tight lid on spending until he reaches 55. His son will have graduated from high school by then, which will free the couple to travel more. Still, Udo says they’ll be careful not to let their annual spending rise to more than 4 percent of their portfolio. Then, at 65, Udo will start collecting Social Security, which should ease the pressure on their nest egg.

“It’s not really stressful at this point, because everything is going really well,” Udo says, alluding to the buoyant stock market. He’s “hoarding cash” on the chance he can buy stocks when prices dip, as he expects they will: “It seems like the stock market valuation is pretty high right now.”

Early retirement isn’t for everyone, Udo says. “But if you really put your mind to it, I think it’s in reach for a lot of people.” He says most people lack the discipline, though. Udo figures he and his wife can adjust, including working part time, if some unexpected expense crops up. Whatever happens, he adds, “I definitely don’t want to go back to working in a corporation.”

The Accountant
Before Sydney Lagier retired in 2008, at 44, she set up an elaborate spreadsheet to take her and her husband to age 100. Every quarter since, the couple, both certified public accountants, review their spending and investments. Eight years later, everything’s on track, despite the global financial crisis. “I figure if I kept my cool during the worst recession of my lifetime, I can probably weather any storm now,” says Lagier, who lives in Redwood City, Calif. She and her husband, who’s also retired, spend just under 3 percent of their portfolio each year. At that pace, she says, their money will carry them through age 102. Lagier declined to give further details on her finances.

Lagier has several safety nets in place, in case some event throws off her planning: The couple try to keep two or three years of cash on hand, enough to be able to ride out a 2008-style meltdown without selling stocks. The equity in their home—worth about 21 times their annual expenses—provides peace of mind. And if all else fails, about 40 percent of their budget is made up of entertainment, clothing, vacations, and eating out, all of which they could cut.

Lagier tried working part time for a while but didn’t enjoy it: “There was so much I wanted to be doing that I wasn’t getting time for.” Now she keeps busy taking piano lessons, exercising, and writing a book about retirement. Her husband is learning the acoustic guitar, and the pair regularly head into San Francisco to listen to live jazz.

The Road Tripper
Justin McCurry is a 36-year-old transportation engineer who hasn’t worked full time in three years. He didn’t want to have a job for 20 more years, he says. “What if I keel over at my desk at 56?” He and his wife, who quit her back-office job at Credit Suisse in February, spend about 3 percent of their $1.5 million portfolio a year.

Their lifestyle in Raleigh, N.C., is comfortable but not cushy. They eat out once a month and get takeout at other times. They’ve paid off their modest home, in a “convenient but lower-middle- to middle-class area in the city.” Their cars “were usually the oldest cars in the parking lot at work.” They don’t clip coupons but do look for sales. “I guess you could say we have inexpensive tastes and desires,” McCurry says. “But we’re not minimalist by any means.”

He has a smartphone, two high-definition TVs, and a PlayStation system. His three kids each have a tablet. “Any kind of electronics we need, we have,” he says, even if some are secondhand.

McCurry does some consulting—“work seems to chase me down,” he says—and makes $1,000 to $2,000 a month blogging about financial independence. The rest of his income comes from a portfolio that’s almost 100 percent stocks. Equities historically provide higher returns than a mix of stocks and bonds, but they’re also more volatile: The value of McCurry’s portfolio shrank $70,000 in one day in June, after Brexit led the markets to swoon. “You do have to be comfortable with, and understand, the potential for huge losses in the stock market,” he says.

McCurry, his wife, and their children recently took their annual summer road trip to Canada in the family minivan. It’s a monthlong adventure they could never indulge in if they were employed full time.

He advises flexibility, even while being disciplined about spending. “It seems like very few people follow the 4 percent rule strictly,” McCurry says. “People tend to spend more when times are good and spend less when times are bad.”