Fact: Some people age three times faster than others.

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The process of ageing is an inevitable part of life for all of us. But why is it that friends of the same age can differ so much in their appearance and health?

Apparently, we some of us can age up to three times as fast as others. Before you run screaming to the bathroom to grab your anti-ageing cream, the study, published in the Proceedings of the National Academy of Sciences, found there were several factors responsible for this.

The US researchers studied a group of 30 year olds and found that their biological ages range from as young as 28 up to 61. Wow.

Related: The areas of your body you didn’t realise were ageing.

The researchers used markers such as blood pressure, lung function, cholesterol, body mass index and DNA integrity to obtain a measurement of a persons biological age.

Random people were then asked to look at photos of the study participants and give an estimate of their age. The results found that people who aged faster mentally were also far more likely to look older in appearance – both to themselves and to other people.

Those who were perceived to have aged significantly were people who were “were less physically able, showed cognitive decline and brain ageing, self-reported worse health, and looked older”. (Post continues after gallery.)

And you might want to hold off before you go cursing your genes for appearing older; the scientists found that only 20 per cent of our rate of ageing can be attributed to our genetics.

One of the study’s authors, Professor Terrie Moffitt from Duke University explaind that the findings give “us hope that medicine might be able to slow ageing and give people more healthy and active years.”

Related: When should I start using anti-ageing products?

Yes, your diet, exercise regime and health habits are responsible for 80 per cent of your body’s ageing. Here’s what they do…

Diet

Diet is a huge contributor to the ageing process with people who enjoyed a high fat, high salt diet much more likely to display signs of ageing in relation to skin tone and elasticity as well as in areas such as weight and muscle tone.

However, it's also interesting to note that people who crash diet regularly also seem to show signs of ageing before those who take a slow and careful approach to weight loss. This is because when we lose (and regain) significant amounts of weight, the elasticity in the skin simply doesn't have time to adjust to the changes and therefore appears saggy and wrinkly.

Genetics

Like a lot of things, we can often thank our genetics for the manner in which we age. Hereditary issues such as dark under eye bags, wrinkles and loss of skin elasticity can all affect the way we appear to other people and are often experienced as a result of our genes.

Related: 7 physical signs of stress you're probably not aware of.

Scientists at the National Institute of Ageing say that studies have so far identified hundreds of genes which are linked to the ageing process and note that often, those who fare well over the years can be seen in similar family groups.

Lifestyle

It's common sense really: Those that lead a healthy lifestyle are far more likely to appear younger than others who share the same age. Not only does lifestyle relate to exercise, but also refers to the way people take care of individual areas of the body such as skin and hair.

Stress

Stress can have an impact on the way people appear. People who lead stressful lives are more likely to show signs of ageing such as fine lines, wrinkles, skin sagging and tiredness before those who have reduced stress levels.

Related: What is preventative botox and is it really necessary?

A study conducted at the Brigham Women's Hospital in Boston found significant links between prolonged exposure to stress and premature ageing.

This is due to the over-release of stress hormones which enter the bloodstream. Over time, this creates an imbalance of hormones which can lead to rapid ageing, heart and cardiovascular issues as well as increasing blood pressure which ages your arteries.

Smoking

Smoking is a huge contributor to premature ageing with regular smokers often appearing a lot older than they actually are. This is because of the effect chemicals and nicotine have on the skin hair and eyes.

Related: This is the one thing proven to help people quit smoking.

Studies have found that cigarettes contain an enzyme which breaks down the elasticity in the skin.