Sitting down too much can age women by 8 years, but not men
Sitting for too long can age women by up to eight years, say scientists.
A study found those who did little exercise and spent ten hours of the day in a chair had cells that were biologically much older.
They had shorter telomeres - tiny caps found on the ends of strands of DNA that have been likened to the plastic tips on shoelaces.
These protect chromosomes from deterioration and progressively shorten with age - increasing the risk of disease.
Dr Aladdin Shadyab of California University in San Diego, said: "Our study found cells age faster with a sedentary lifestyle.
"Chronological age doesn't always match biological age."
Previous research has found sitting may be particularly harmful for women by raising their risk of cancer.
Those who sat more than six hours a day were ten percent more likely to develop the disease compared to those who sat less than three hours a day.
Most men in that study of almost 150,000 people did not appear to be at an increased risk from cancer from too much sitting.
The latest findings published in the American Journal of Epidemiology are based on almost 1,500 elderly women aged 64 to 95.
They showed those who managed less than 40 minutes of moderate-to-vigorous physical activity per day and who remained sedentary for over ten hours had shorter telomeres.
As a cell ages its telomeres naturally shorten and fray but health and lifestyle factors - such as obesity and smoking - may accelerate that process.
Shortened telomeres are associated with cardiovascular disease, diabetes and major cancers.
Dr Shadyab and his researchers believe they are the first to objectively measure how the combination of sedentary time and exercise can impact this sign of ageing - or biomarker.
The participants were from the larger Women's Health Initiative (WHI) - a long-running study in the US investigating the causes of chronic diseases in post menopausal women.
They completed questionnaires and wore an accelerometer device on their right hip for seven consecutive days during waking and sleeping hours to track their movements.
Dr Shadyab said: "We found women who sat longer did not have shorter telomere length if they exercised for at least 30 minutes a day - the national recommended guideline.
"Discussions about the benefits of exercise should start when we are young and physical activity should continue to be part of our daily lives as we get older - even at 80 years old."
He said future studies will examine how exercise relates to telomere length in younger populations and in men.
Doctors have warned sitting is the new smoking - with the average Brit now spending a staggering 8.9 hours every day sitting down.
That might be at work, in a car or on the sofa in front of the TV. Add another seven hours sleeping and that means most of us spend just one third of our time on our feet.
Those prolonged periods of inactivity increase our risk of obesity, but they also cause a staggering list of other conditions.
This includes heart disease, diabetes, colon cancer, muscular and back issues, deep vein thrombosis, brittle bones, depression and even dementia.
Experts are now describing sitting as 'the new smoking', a ticking time bomb of ill health just waiting to explode.
The World Health Organisation has identified physical inactivity as the fourth biggest killer on the planet, ahead of obesity.
It now costs the UK economy more than £1billion every year in sick days due to back, neck and muscle problems and that figure is still rising.