There are many, many good reasons to quit smoking already - but Nature provides a new and very compelling one: "Like the plastic tips on the ends of shoelaces, telomeres help to protect genes against wear and tear. ... Tim Spector [and] his team have shown that telomeres shrink dramatically in patients who are obese or heavy smokers. ... Heavy smokers, who consumed a pack a day for 40 years, showed seven years of extra biological ageing. Researchers already knew that smoking and obesity could cause a kind of stress in cells that produces reactive chemicals, which in turn are known to wear away telomeres. Spector's large study looks directly at this effect to quantify just how much a cigarette can age you."
Link: http://www.nature.com/news/2005/050613/full/050613-3.html