Break out the balloons and confetti. The nation’s population reached a new milestone Tuesday -- 300 million -- according to the U.S. Census Bureau.

If the idea of living in an increasingly crowded country doesn’t put you in a celebratory mood, consider this: The dramatic rise in population over the last century has been accompanied by an even more phenomenal rise in life expectancy.

When the U.S. population reached 100 million in 1915, the average lifespan was 54 years. When we hit 200 million in 1967, it was around 70.

Today, the average lifespan of someone living in the U.S. is just months shy of 78, and there is little reason to think that we won’t continue to push the life expectancy envelope.

“Life expectancy worldwide has been rising pretty steadily since 1840, at a rate of about two years per decade,” Daniel Perry, who is executive director of the Alliance for Aging Research, tells WebMD. “In 1840, the longest-living people in the world were women in Sweden, and they lived an average of 45 years.”

Death Quiz: How Long Will You Live Past 50?

100 and Beyond

Some experts on aging say that within 50 years, the average person living in an industrialized nation with good access to health care will live to be at least 100.

James Vaupel, who directs the laboratory of survival and longevity at Rostock, Germany’s Max Planck Institute, has written that advances in the prevention, diagnosis, and treatment of age-related diseases, such as heart disease and cancer, will usher in an age where every second child born in the industrialized world has an even chance of reaching the century mark.

“Although the belief that old-age mortality is intractable remains widespread, life expectancy is not approaching a limit,” he writes.

During the first half of the 20th century revolutionary advances in medicine and public health were responsible for raising the average life expectancy in the U.S. by more than 20 years -- from age 47 in 1900 to age 68 in 1950.

According to the CDC, the 10 greatest medical and public health achievements of the 20th century were:

Vaccination against disease, resulting in the eradication or elimination of major diseases of the early 20th century, such as smallpox and polio Control of infectious disease through improved sanitation, clean water sources, and the introduction of antibiotics Improvements in motor-vehicle safety Improved workplace safety Improved food safety Decline in deaths from heart disease and stroke Smaller families with longer birth intervals due to family planning Better prenatal care Fluoridation of drinking water Public health efforts to reduce smoking

U.S. Death Trends Are Shifting

Heart Disease, Smoking

Winifred Rossi is deputy director of the Geriatrics and Clinical Gerontology Program at the National Institute on Aging.

“In 1900 infant mortality, infectious disease, pandemics, and war were the big killers,” she tells WebMD. “People tended to die quickly at young ages. The big killers today are chronic diseases of aging like heart disease and cancer.”

The biggest single factor in the increase in life expectancy during the latter half of the 20th century and beyond has been the improvement in the prevention, diagnosis, and treatment of cardiovascular disease, the experts agree.

In just the last 25 years, there has been an almost 50 percent reduction in deaths from stroke and heart attacks in the U.S., Perry says.

Cancer deaths are also declining, driven largely by public health efforts to educate Americans about the dangers of smoking. Lung cancer deaths among men have been declining since the mid-1970s, and increases among women have begun to stabilize.

Dramatic reductions in infant mortality and easier access to emergency care have also helped increase life expectancies in the last three decades of the 20th century.

Why Do Women Live Longer Than Men?

Living Longer and Better

Americans are living longer but are they living better? Are the extra years worth it in quality-of-life terms? Or are they filled with avoidable suffering related to failing health?

Certainly, most people who make it to their eighth decade experience age-related health challenges. The average 75-year-old has three chronic health conditions, and the list of chronic diseases that are linked to aging seems endless. Heart disease, stroke, cancer, diabetes, Alzheimer’s and Parkinson’s disease, and arthritis are just a few.

But there is some intriguing clinical evidence that in spite of their health problems, older people today really are happier, healthier, and are functioning better than their parents or grandparents.

In 2001, Duke University researchers reported a dramatic decline in disability among older Americans, based on their population-based study of functionality from the early 1980s to 1999.

This occurred despite a big increase in elderly population in the U.S. during the period.

“Older people today are able to remain functionally independent much longer than in the past,” Perry says.

And research from the CDC and the Merck Institute of Aging and Health suggests that older Americans actually experience less depression and have a better mental outlook than their younger counterparts.

“One theory is that their life experience helped them deal with problems better and made them more mentally resilient,” says Maggie Moore, of the CDC’s Healthy Aging Program. “Older people have seen a lot, and the attitude might be, ‘This isn’t the worst thing that has ever happened.’”

Will Obesity Shorten the American Life Span?

Staying Young as You Age

So how do you maximize your chances of dancing a jig at your 90th birthday party? Chances are you already know the answer, but let’s review one more time.

Eat a healthy diet with plenty of fruits and vegetables; stay physically fit and active; don’t smoke; don’t drink too much alcohol; get plenty of sleep; and see your doctor regularly.

In other words, do all you can to prevent the avoidable chronic diseases of aging, the experts agree.

“Some people have lived as long as 120 years, so we know that this is possible for our species,” Perry says. “There is no obvious barrier to living well beyond 100.”

Lessons From the Earth's Elders

By Salynn Boyles, reviewed by Louise Chang, MD

SOURCES: Daniel Perry, executive director, Alliance for Aging Research, Washington. Maggie Moore, MPH, Healthy Aging Program, CDC, Atlanta. Winifred Rossi, MA, deputy director, Geriatrics and Clinical Gerontology Program, National Institute on Aging, NIH, Washington. CDC, “10 Great Public Health Achievements," MMWR, April 2, 1999. National Center for Health Statistics, life expectancy tables, 2005. “The Remarkable Rise in Life Expectancy and How it Will Affect Medicine,” James W. Vaupel, Bundesgesundheitsblatt Gesundheitsforschung Gesundheitsschutz, May 2005. WebMD Medical News: “Cancer Deaths Continue to Drop.” “The State of Aging and Health in America 2004,” CDC and the Merck Institute of Aging and Health.