You're only as old as you feel, the saying goes, and it turns out that some of us seem to be ageing more slowly than we were two decades ago, according to new research.

That doesn't mean you can wait for more than a year between celebrating each birthday, but there's evidence that biological ageing – the wear and tear showing on the cells in our bodies – appears to be slowing down in some cases.

Biological age is a good indicator of the health of a population, and if its pace slows, that means we're not just staying alive for longer, we're also living healthier lives for longer.

The team of researchers looked at health and nutritional data on 21,575 people aged under 80 in the US between 1988 and 2010, examining data for metabolism, inflammation, organ function, blood pressure, and breath capacity.

"This is the first evidence we have of delayed 'ageing' among a national sample of Americans," says the senior author of the new study, Eileen M. Crimmins from the University of Southern California (USC).

Among the measurements made by researchers were levels of haemoglobin, total cholesterol, creatinine (related to kidney function), alkaline phosphatase (related to liver disease), albumin and C-reactive protein in the blood.

These can all indicate how fit and healthy a body is, and the researchers found full sets of data for 70 percent of their sample.

There are also plenty of outward signs – as we get older, not everyone is blessed with the same level of health and well-being. That's biological ageing at work.

The study found that biological age is lower for recent periods across all age groups, but the difference varies based on age and gender. The scientists think that changes in smoking, obesity, and medication use are partly the reason.

Men aged 60-79 showed the biggest improvement, with an average biological age drop of more than 4 years between the 1990s and the 2000s – so men in this age group were biologically healthier overall than men in the same age group a decade before.

The researchers emphasise that efforts need to be made to keep these kind of improvements consistent over all age groups and genders, otherwise certain sections of the population could miss out on an improved quality of life.

"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 one of the team, Morgan E. Levine from Yale University in Connecticut.

Biological ageing is important for society as a whole because it means the population is living longer before a higher risk of serious illness and disability kicks in.

And although this study concentrated solely on people in the US, it gives all of us hope that improvements in medical care and healthier lifestyles are having a positive effect on the quality of life in our later years – not just how long we live for.

In the US, for example, women can expect to live another 20 years once they reach 65. On average though, only 11 will be disease free.

And how can we slow down our own biological ageing? There's no magic formula, just the usual tips: don't smoke, eat healthily, and keep exercising.

"Lifespan extension accomplished through a deceleration of the ageing process will lead to lower health care expenditures, higher productivity and greater well-being," says Levine.

The research has been published in Demography.