Background: Marijuana is the most commonly used illicit drug in the United States. There has been a rapidly changing marijuana policy environment and increased acceptability related to marijuana in the United States. How the changing environment will potentially influence adolescents age of initiation remains unknown. While much of extant literature has primarily focused on current marijuana use, less is known about age of first use. This study examined trends in adolescents' age of first marijuana use in the United States. Method: Data were drawn from the 1991 to 2017 Youth Risk Behavior Survey. We used linear regression for age of first use as a continuous variable, and logistic regression for marijuana use before the age of 15 years as a dichotomous variable. Joinpoint regression analysis identified where significant changes in trend occurred. Results: Results showed that the mean age of first marijuana use increased significantly between 1991 and 2017. Males had a younger age of first use than females. Between 1991 and 1997, there was an increasing trend in the prevalence of marijuana use before the age of 15 years in all adolescents and in subgroups for males, females, all races, and 9th and 10th grades. After 1997, a significant downward trend was found in all adolescents. Conclusions: Our results do not indicate an overall decreasing trend of age of first use among the general adolescent population. Results show a downward trend in the use of marijuana before the age of 15 years since 1997.