Research is limited regarding adolescents’ and young adults’ (AYA) patterns, methods of, and reasons for cigarette or e-cigarette cessation. Further, while adults may try to use e-cigarettes to quit combustible cigarettes, little is known about how adults then quit e-cigarettes. This study utilizes a national, cross-sectional online survey of 6131 diverse participants aged 13–40 years to examine reasons for quitting e-cigarettes or cigarettes, quit methods, and quit attempt outcomes among AYAs and adults. In our sample, 3137 (51.2%) had ever used an e-cigarette, of whom 2310 (37.7%) were aged 13-24 years and 827 (13.5%) were 25-40 years old; 2387 (38.9%) had ever used a combustible cigarette (1440 [23.5%] were 13-24 years old and 947 [15.4%] were 25-40 years old). Among e-cigarette ever-users, 39.4% of 13–24-year-olds intended to quit in the next 6 months, and 36.9% had a serious plan for quitting in the next 30 days; 25.2% wanted to decrease the amount they used while 34.8% wanted to quit completely. Similar rates were found among e-cigarette ever users aged 25–40 years, as well as past 30-day e-cigarette users, cigarette ever-users, and past 30-day cigarette users across all ages. “Cold turkey” (41.0%) followed by “tried to cut down slowly by vaping/smoking less often or fewer puffs” (25.5%) was the most common quit method among e-cigarette ever-users and cigarette ever-users of all ages. Further study of effective tobacco cessation methods to help both AYAs and adults successfully quit nicotine, whether from cigarettes or e-cigarettes, is urgently needed.