Abstract Objective To assess the long-term impact of the increase in age-of-sale of cigarettes from 16 to 18 in England in October 2007. Methods Data were collected between November 2006 and September 2018 on 252,601 participants taking part in a nationally representative survey of adults aged 16+ in England, the Smoking Toolkit Study (STS). We assessed the impact of the introduction of the increase in age-of-sale on prevalence of ever smoking, current smoking, and quit attempts, among 16–17 year olds compared with 18–24 year olds. Results Following the increase in age-of-sale, there was a declining trend in ever smoking that was greater among 16–17 year olds than 18–24 year olds (OR 0.990 versus OR 0.993; p = 0.019). Data on current smoking were insensitive to detect a difference between the age groups in a step-level change or change in trend following the increase in age-of-sale (Bayes factors (BFs) 0.75 and 2.10). Data on quit attempts were also insensitive to detect a change in trend (BF 0.71), and despite a greater step-level decline among those aged 16–17 (OR 0.311 versus OR 0.547, p = 0.025), quit attempts remained higher among those aged 16–17. Secondary analysis indicated that post-policy change, trends in current and ever smoking were linear for 16–17 year olds but quadratic for 18–24 year olds (slowing decline). Conclusion There is some evidence from an assessment of long-term trends in the Smoking Toolkit Study that the increase in legal age-of-sale of cigarettes in England was associated with a greater long-term decline in ever smoking among those aged 16–17 compared with those aged 18–24.