BackgroundTobacco 21 (T21) laws, which raise the minimum legal age of sale of tobacco products to 21, have been proposed and implemented in states and cities across the USA. However, limited data are available on the effect of T21 laws on youth tobacco purchasing behaviours and access to tobacco products.MethodsParticipants in a population-based prospective cohort in southern California completed questionnaires before (n=1609, age=18–19 y) and after (n=1502, age=19–20 y) T21 was implemented in California (June 2016). We examined the prevalence of past 30-day cigarette and e-cigarette use, and among past 30-day users, purchase location of tobacco products before (pre-) versus after (post-) T21. We also examined whether, post-T21, participants were refused purchase of tobacco products due to their age, and the perceived relative ease of purchasing cigarettes and e-cigarettes (vs pre-T21).ResultsNegligible changes in cigarette and e-cigarette use were observed pre-T21 versus post-T21. At both time points, the majority of past 30-day users purchased cigarettes from gas stations and e-cigarettes from vape shops. Post-T21, the proportion of participants who reported purchasing cigarettes at gas stations decreased. Post-T21, most past 30-day cigarette or e-cigarette users were not refused purchase of cigarettes (65.4%) or e-cigarettes (82.0%) in the past 30 days, despite being under 21; half of the participants felt it was harder to purchase cigarettes (54.3%) and e-cigarettes (43.6%) post-T21.ConclusionPost-T21, few participants were refused purchase of any tobacco product, despite the illegality of such sales. Better enforcement of T21 is needed to improve the efficacy of T21 legislation.