Fact
The majority of smokers begin before the age of 18 (nearly 87% before age 18, and nearly 94% before age 20).
Source 1:
"The Health Consequences of Smoking—50 Years of Progress. A Report of the Surgeon General." U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health. 2014. Report.
http://www.surgeongeneral.gov/library/reports/50-years-of-progress/full-report.pdf