Even when people are born in the same year, they can often look very different as they age. Some appear younger, and others older than their chronological age. But it's not just looks - earlier this year researchers were able to show that people do indeed age at different rates. Now another group of scientists claims to have developed a way of testing how well a person is aging, and could help health professionals to predict who might be at a higher risk of dementia or other age-related conditions.

“We use birth year, or chronological age, to judge everything from insurance premiums to whether you get a medical procedure or not,” explained James Timmons, who led the study, published in Genome Biology, in a statement. “Most people accept that all 60 year olds are not the same, but there has been no reliable test for underlying 'biological age'.”

That is, according to the researchers, until now. They say that they have determined a "gene signature" present in the blood that indicates if an individual is aging “healthily.” They did this by looking at the activity of around 54,000 genes in healthy 25 to 65 year olds, before narrowing it down to 150 genes that could reliably distinguish between the old and young people in the study, and also predict risk of cognitive decline. This "signature" was then used to develop what the researchers are calling a “healthy age gene score,” which they compared between individuals to show how a higher score was associated with better health.

“Our discovery provides the first robust molecular 'signature' of biological age in humans and should be able to transform the way that 'age' is used to make medical decisions,” added Timmons, who is based at King’s College London. “This includes identifying those more likely to be at risk of Alzheimer's, as catching those at 'early' risk is key to evaluating potential treatments.”

To test the “healthy age gene score,” the scientists used data from a separate study, of Swedish men who were aged 70 in 1992, which had been collected over 20 years. They found that the gene score varied widely across the men, and demonstrated that it was linked to long-term health. Those with a higher score had greater organ – especially renal – function after 12 years, and better cognitive health. In particular, they were able to show how those with Alzheimer’s had an altered gene signature in their blood.

It is hoped that the use of such a predictive test could help with clinical trials that attempt to prevent the development of dementia, as currently we can only attempt to reverse, slow or halt it. A more accurate measure of a person’s biological age might also have implications for how doctors treat patients, when people receive their pension, or even for insurance premiums.