Association between age at first reported e-cigarette use and subsequent regular e-cigarette, ever cigarette and regular cigarette use.

"Declaration of interests All authors report receiving grants from the National Prevention Research Initiative during the study. The authors have no conflicts of interest."

"The research was supported by a grant MR/J000264/1 from the UK Medical Research Council/National Preventive Research Initiative. The UK Medical Research Council had no role in the design and conduct of the study; collection, management, analysis and interpretation of the data; preparation, review or approval of the manuscript; and the decision to submit the manuscript for publication. C.J.A. is additionally supported by the NIHR Manchester Biomedical Research Centre and the NIHR Greater Manchester Patient Safety Translational Research Centre. The authors thank the trial steering committee (Professor Amanda Amos, Dr Ian Cameron, Professor Christopher Gidlow, Professor Thomas Webb) for advice on measuring e‐cigarette use. All available data can be obtained by contacting the corresponding author; the study team will retain exclusive use until the publication of major outputs. The authors of this article affirm that the manuscript is an honest, accurate and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained. All authors report receiving grants from the National Prevention Research Initiative during the study. The authors have no conflicts of interest."