A new method for measuring biological age - the decline in function of organs as we grow older - has been used for the first time in young adults.
Each factor was weighted equally to ensure no bias was given to one measure over any other, though the authors acknowledge that this is something that will require more research for future studies.
The first measures for this study were taken when the group were aged 26, with follow up screenings at ages 32 and 38. The biological ages of the 38-year-old participants ranged from under 30 to over 60, based on the 18 factors being measured.
The team, from Duke University, also measured the 'pace of ageing' in the study group. This was calculated using the data from ages 26, 32 and 38 by drawing a 'line of best fit' for each of the 18 factors and then summing the slopes of those lines to produce an estimate for the pace of ageing.
Most participants were found to be ageing at the expected rate of around 1 biological year per chronological year, but some had values as high as 3 and others even had zero or negative values.
The participants with a negative pace of ageing are of particular interest because they may reveal molecular and behavioral pathways linked to rejuvenation.
A further test was carried out using photographs of the participants' faces. A group of undergraduate students at Duke University were shown images of each of the participants and asked to record what age they thought that person was. Once again, those with a higher biological age were recorded as older by the students.
This is an important step in understanding the ageing process, since many chronic diseases associated with old age begin to form at a much younger age.
By studying young adults the team hope to be able to identify the first signs of decline in organ systems before the onset of disease, and therefore provide earlier treatment and prevention measures.
The ultimate goal of the study is to be able to intervene in the aging process as early as possible, with future studies on young children already planned.
As Dan Belsky, the lead author in the paper, puts it, "To prevent multiple diseases simultaneously, ageing itself has to be the target. Otherwise, it's a game of whack-a-mole..."