Everybody wants to live longer, and to help turn back the clock scientists are working on drugs, stem cell treatments and caloric restriction. But how well are these efforts paying off? A comprehensive new study from Yale University and the University of Southern California (USC) has found that Americans seem to be aging more slowly now than they were 20 years ago.

If someone asks you your age, the number on the tip of your tongue is probably your chronological age – the amount of years that have passed since you were born (or at least, the amount you're willing to admit). But due to a massive range of lifestyle and genetic factors, everyone physically experiences aging at a different rate, and this is known as your biological age.

It's this second number that the new study focused on. The researchers gathered health and nutrition data from over 21,500 people, calculated their biological age and compared it to their chronological age, to see how the difference might have changed between 1988 and 2010. To determine biological ages, the team used a wide range of health indicators, such as blood pressure, breath capacity, and the amount of certain markers in the blood, including hemoglobin, cholesterol, creatinine, alkaline phosphate, albumin and C-reactive protein.

Across the board, the researchers observed an overall trend towards a lower biological age. In that time, the difference gains between patients' biological and chronological ages seemed to be higher for older people, and for men compared to women.

"While improvements may take time to manifest, and thus are more apparent at older ages, this could also signal problems for younger cohorts, particularly females, who — if their improvements are more minimal — may not see the same gains in life expectancy as experienced by the generations that came before them," says Morgan Levine, lead author of the study.

The researchers say this is the first evidence of delayed aging in a national sample, and the positive results might be at least partly attributed to reductions in smoking, improvements to medicine, and healthier behaviors.

It's well documented that the average life expectancy has been on the increase for decades, but the new study shows this isn't just a case of being able to keep sick people alive for longer. Our healthspan is improving too – the proportion of our lives we enjoy living in good health.

"Life extension without changing the aging rate will have detrimental implications," says Levine. "Medical care costs will rise, as people spend a higher proportion of their lives with disease and disability. However, lifespan extension accomplished through a deceleration of the aging process will lead to lower health care expenditures, higher productivity and greater well-being."

The research was published in the journal Demography.

Source: USC