Smokers who quit by around age 40 can stave off an early death, according to a landmark study that fills key gaps in knowledge of smoking-related health ills.

While smokers who never stop lose about a decade of life expectancy, those who quit between ages 35 and 44 gained back nine of those years, the study found.

Moreover, the benefits of dropping the habit extend deep into middle age. Smokers who quit between 45 and 54 gained back six otherwise lost years, and those who quit between 55 and 64 gained four years.

Quitting young, before age 35, erased the entire decade of lost life expectancy.

The message: It's never too late to quit, even for heavy smokers with decades of puffing behind them.

But younger smokers should not be lulled into thinking they can smoke until 40 and then stop without consequences, said Prabhat Jha, an epidemiologist at the Center for Global Health Research in Toronto. Jha led the new study, published online Wednesday in the New England Journal of Medicine.

That's because the risks of lung cancer and other respiratory diseases linger for years after stubbing the last butt.

Most of the gains in life expectancy come because the twin risks of heart disease and stroke quickly drop after smoking ends. Both diseases occur as the byproducts of tobacco smoke trigger clotting in the arteries, a process that can rapidly reverse.

Damage to the lungs, meanwhile, takes longer to heal. "The risk for lung cancer doesn't disappear and the risk of respiratory disease doesn't disappear" in former smokers, said Jha. "But the acute risk for heart attack or stroke pretty much disappears."

While the study delivered some good news for quitters, it also hammered home the message that continuing to smoke carries grave risks.

Current smokers in the study died early at a rate triple that of people who never smoked. And few smokers reached age 80. Just 38 percent of female smokers and 26 percent of male smokers hit that milestone, while 70 percent of women who never smoked and 61 percent of men who never smoked did.

The study linked surveys of 217,000 adults collected for the federal National Health Interview Survey between 1997 and 2004 to cause-of-death records in the National Death Index.

Previous long-term studies of smokers followed specific groups who were likely healthier than the general population, such as nurses or physicians, said Jha, marking the new study as more representative of the risks of smoking — and the benefits of dropping the habit — across the entire nation.

A second report, also published in the New England Journal of Medicine, showed that smoking-related deaths among women have soared in recent decades. For the first time since research on smoking and health began in the 1950s, the rate of smoking-related deaths is now nearly equal between male and female smokers.

Women took to cigarettes in large numbers only after World War II, lagging behind men by about 20 years. The consequences of that shift are just now reaching women in their mid-50s and older. Meanwhile, lung cancer risk in male smokers leveled off in the 1980s.

"As the Mad Men generation has matured, the risks in women who smoke continue to increase," said Michael Thun of the American Cancer Society, lead author of the second report, which drew on that group's Cancer Prevention Study. That study tracked 1.2 million men and women through 2010.

"We used to think women were at less risk of illness from smoking," said Steven Schroeder, a smoking and health expert at the University of California, San Francisco, who was not involved in either study. "That doesn't seem to be true now. My guess is that early women smokers didn't smoke as much as men because there was some stigma to it. Now they're smoking as many cigarettes as men are."

In other words, he said: "Smoke like a man, die like a man."

In the American Cancer Society study, men and women smokers both experienced a 25-fold higher risk of dying from lung cancer over people who never smoked. Risk of death from other lung diseases also soared about 20-fold in smokers.

About 45 million Americans smoke, including 21 percent of men and 17 percent of women, according to a 2010 report from the Centers for Disease Control and Prevention. Smoking has been steadily falling in popularity since the 1970s for men and the 1980s for women. Still, cigarette smoking is the nation's leading cause of preventable deaths, the CDC says, killing about 443,000 people each year.