Today, smoking is widely considered to be one of the worst things you can do for your health.It permanently damages your lungs, raises your risk of getting nearly 40 different cancers, and drastically reduces life expectancy.Quitting can be a difficult, painful and lengthy process. And for some, it's a challenge that feels futile – around one-third of smokers believe stopping won't make a difference because the 'damage is already done'.But experts speaking to the Daily Mail now say that, in fact, the opposite is true.Not only does quitting smoking drastically reduce the risk of dying from lung or heart disease, giving up the habit before a certain age can actually reverse any harm already done.People who stop smoking by age 35 will have roughly the same chance of dying from cancer as those who never smoked a day in their lives, pioneering research has shown.Quitting by 44 evades around 90 per cent of health risks. And even giving up by age 54 can avoid 78 per cent of the risk of dying from smoking-related cancer.It sounds astonishing, given the shocking health impacts that smoking can have on the body. Smoking permanently damages your lungs, raises your risk of nearly 40 different cancers, and drastically reduces life expectancy But experts say the growing evidence should encourage smokers to quit while they can still reverse much of the damage done by the habit.'Smoking is the biggest killer in this country,' says Sir Richard Peto, emeritus professor of medical statistics and epidemiology at the University of Oxford.'You can add up every reliably known cause of cancer in this country except for smoking, and it still won't come close to causing the same number of deaths.'But what we now know to be absolutely definite is that if you stop smoking at 30, you avoid nearly all of the risk of getting killed by it. Stopping works – even well into middle-age.'Our bodies can repair themselves. But you have to allow them to.'While smoking rates among the young have dropped in recent decades – just 2 per cent of teenagers today have picked up the habit – more than five million of us still light up a cigarette every day.Research shows that these life-long smokers live 11 years on average less than non-smokers.Three in five smoking-related deaths will be due to lung cancer – which kills more than 35,000 Britons each year, and is 30 times more likely to strike smokers than non-smokers.