Despite laws prohibiting sale of e-cigarettes to individuals aged less than 21 years, many underage young adults purchase e-cigarettes from retail stores, which may increase likelihood of continued use due to a greater access to vaping products and exposure to point-of-sale marketing. Data are from a prospective cohort of young adults aged 18-20 years in Los Angeles who had ever used e-cigarettes at baseline (N = 1,029). We evaluated the association of underage e-cigarette purchasing behavior (owned and purchased vs. owned but never purchased vs. never owned an e-cigarette) with subsequent vaping frequency, intensity, and dependence symptoms one year later, adjusting for vaping behaviors prior to baseline; additional models evaluated whether associations differed by purchase location or product type. At baseline, 332 (32%) had purchased e-cigarettes while underage, 227 (22%) owned but never purchased e-cigarettes themselves, and 470 (46%) never owned an e-cigarette. Compared to never owning e-cigarettes, those who had purchased their own e-cigarettes vaped more days in the past month (rate ratio [RR] = 2.97; 95% confidence interval [CI]: 2.15-4.09), had more vaping episodes per day (RR: 2.58; 95% CI: 2.12-3.14), vaped more puffs per vaping episode (RR: 1.90; 95% CI: 1.61-2.23), and had greater odds of dependence (odds ratio: 3.68; 95% CI: 2.51-5.40); elevated estimates were also observed for those who owned but never purchased e-cigarettes (vs. never owned). Vaping dependence was greatest among participants who purchased JUULs or other pod-mods. Participants who purchased e-cigarettes underage subsequently vaped more intensely and had greater vaping dependence. Regulations that reduce underage retail access to e-cigarettes may help prevent vaping progression among those most at risk of dependence.