Writing in the Aug. 18 edition of the Journal of the American Medical Association, Adam Leventhal of the University of Southern California and his team survey teens  about past and recent e-cigarette use in terms of potential recruitment to combustible tobacco use.
Their survey employs yes/no responses that do not differentiate experimentation from continuing use. The findings show that 768 of the teens tried combustible tobacco first, 3.5 times as many as the 222 teens who initiated nicotine use with e-cigarettes. Furthermore, 49.1 percent of the e-cigarette initiators admitted to a history of substance abuse, compared to 15.0 percent of those with no such history
These data show that those inclined to experiment with a chemical substance are more likely to initiate e-cigarette use than those not inclined to such experimentation. In other words, the data reported in this study do not support the suggestion  that preventing use of e-cigarettes by minors would in any way influence the number of teens experimenting with nicotine delivery products.
More research is needed to address the public-health impact of e-cigarette use by teens. Such research should address experimentation versus consistent and continuing use, use of zero-nicotine e-cigarettes and a wider array of potential confounders. 
- “survey teens”: http://jama.jamanetwork.com/article.aspx?articleid=2428954
- “support the suggestion”: http://jama.jamanetwork.com/article.aspx?doi=10.1001/jama.2015.8382
- “potential confounders.”: http://www.mdpi.com/1660-4601/12/5/5439