Scientists have developed a new molecular test which can tell how well a person is aging. Assessing someone's biological age, they say, is more revealing than their birth date.

The seven-year study was a collaborative effort between King's College London, Karolinska Institutet in Sweden, and Duke University, which used a process called RNA-profiling to measure and compare gene expression in thousands of human tissue samples.

Instead of looking for genes that are associated with disease or extreme longevity, the researchers found that the "activation" of 150 genes in the blood, brain, and muscle tissue were a sign of good health at age 65.

Using the genes as a guide, the researchers created a formula for healthy aging, which can tell how well a person is aging in comparison to others born the same year.

The researchers tried out their test on a group of 70-year-old Swedish men, and found that a person's biological age is separate and distinct from their chronological age.

The test accurately determined which men were aging well, and was able to predict who would die in the next few years.

"You could actually pick out people who had almost no chance of being dead, and you have people who had an almost 45 percent chance of being dead,"  Professor James Timmons told the BBC News.

"Given the biological complexity of the aging process, until now there has been no reliable way to measure how well a person is aging compared with their peers," said Timmons.

"Physical capacity such as strength or onset of disease is often used to assess 'healthy aging' in the elderly but in contrast, we can now measure aging before symptoms of decline or illness occur."

A low score correlated with cognitive decline, suggesting that the test could one day become a simple blood test that would predict those most at risk for dementia, including Alzheimer's disease.

The researchers say theirs is the first practical and accurate test to reveal the rate at which individuals age.

There could be numerous practical applications to the program, from personal decisions on how much money to save for one's old age, to deciding if people who are chronologically old are still biologically young enough to donate organs.

However, the study doesn't provide any answers about how to slow biological aging.

"We now need to find out more about why these vast differences in aging occur, with the hope that the test could be used to reduce the risk of developing diseases associated with age," Timmons said.

"It looks like from the age of 40 onwards you can use this to give guidance on how well an individual is aging," Timmons told the BBC.