"The younger and more often teens use marijuana, the more likely they are to engage in other substance use and the higher their risk of developing a substance use disorder. Among high school students, 7.5 percent used marijuana for the first time before the age of 13. CASA’s analysis of national data finds that the average age of initiation of marijuana use among high school students is 14.3 years old. Compared to those who began using marijuana after age 21, those who first used it before age 15 are:
More likely to have ever smoked a cigarette (93.3 percent vs. 86.4 percent);
More than twice as likely to have ever misused controlled prescription drugs (56.5 percent vs. 22.9 percent); and
Two and a half times as likely to have ever used other illicit drugs (70.2 percent vs. 27.8 percent)."
"Adolescent Substance Abuse: America's #1 Public Health Problem," National Center on Addiction and Substance Abuse at Columbia University, June 2011, p. 27.