Saving 15 percent for a purpose that is decades away requires commitment, especially when it’s not all you’ll need to save: For example, it doesn’t include money for a rainy-day fund, a home’s down payment, a car, a child’s education, a vacation, a wedding, a bar mitzvah, a sweet 16 or anything else. All of that comes on top of the retirement savings. Yet in Mr. Bernstein’s view, that 15 percent is a bare minimum. “You’ve got to accumulate enough money on your own to replace your income one day,” he says. “You’re much better off if you start doing that early.”

Older people who didn’t save enough when they were young need to save more than 15 percent now, if they can. Often, they can’t, which is why so many people in the work force face serious problems, and why Mr. Bernstein is appealing to younger people who can still help themselves.

Note, for a moment, what Mr. Bernstein isn’t even attempting to do. He isn’t advising baby boomers — as a generation, anyway — because he isn’t optimistic about boomers’ overall prospects.

“About one-half of boomers are already in trouble,” he says. “They haven’t put enough money away and it’s going to be hard for them to do it now.” His back-of-the-envelope estimate of the current working population’s lack of preparedness isn’t out of line with the assessments of researchers who’ve spent considerable time on the subject. The Center for Retirement Research at Boston College, for example, puts the percentage of all households at risk “of being unable to maintain their pre-retirement standard of living” at 50 percent. The nonpartisan Employee Benefit Research Institute puts the proportion of baby boomers at risk of running out of money in retirement at 40 to 50 percent.

It’s never too late to improve your individual situation — and public policy solutions may become more realistic if enough people get involved. But whatever policy shifts are possible in America, it’s still extremely likely that most people will need to rely primarily on themselves to finance a comfortable retirement. Hard as that may be, it’s easier if you start early.

Mr. Bernstein, who lives in Portland, Ore., switched to finance and writing after a career as a neurologist. He acknowledges that he’s well off himself; he says he wants to help others who aren’t.

Saving can be very tough, he says, especially when you’re just starting out, so he provides suggestions on how to manage a limited income. They include standard measures like cutting back on lattes, expensive cable TV and cellphone packages, and purchases of high-end sneakers, restaurant meals and the like. In the end, it’s an individual choice, but you may have to give up some things you want.