Fact
Nearly 95% of regular smokers start by the age of 21.
Source 1:
The Health Consequences of Smoking—50 Years of Progress: A Report of the Surgeon General. National Center for Chronic Disease Prevention and Health Promotion (US) Office on Smoking and Health. Atlanta (GA): Centers for Disease Control and Prevention (US); 2014.
https://www.ncbi.nlm.nih.gov/books/NBK294302/table/ch13.t2/?report=objectonly