"21st-Century Hazards of Smoking and Benefits of Cessation in the United States"
NEJM Jan 24
http://www.nejm.org/doi/full/10.1056/NEJMsa1211128?query=featured_home
"CONCLUSIONS

Smokers lose at least one decade of life expectancy, as compared with those who have never smoked. Cessation before the age of 40 years reduces the risk of death associated with continued smoking by about 90%."