"The illicit drugs with the largest number of recent initiates in 2016 were marijuana (2.6 million new users), prescription pain relievers (2.1 million new misusers), prescription tranquilizers (1.4 million new misusers), prescription stimulants (1.4 million new misusers), hallucinogens (1.2 million new users), and cocaine (1.1 million new users). In addition, there were 4.6 million new users of alcohol, 1.8 million people who tried a cigarette for the first time in the past year, and 1.2 million people who first used smokeless tobacco in the past year.34
"Figure 12 provides an overview of the average age at first use (or first misuse for prescription drugs) in 2016 among recent initiates aged 12 to 49. For many substances, the average age at initiation in 2016 was younger than age 20, with average ages of 17.4 years for alcohol, 18.0 years for cigarettes, 18.2 years for inhalants, 19.3 years for marijuana, and 19.6 years for any hallucinogen. However, some substances had older average initiation ages, such as methamphetamine (24.6 years) and heroin (25.5 years). The average ages at initiation for prescription drug misuse were in the early to mid-20s (23.9 years for prescription tranquilizers, 24.3 years for prescription stimulants, 24.4 years for prescription pain relievers, and 24.8 years for prescription sedatives)."
Lipari, R. N., Ahrnsbrak, R. D., Pemberton, M. R., & Porter, J. D. (2017, September). Risk and protective factors and estimates of substance use initiation: Results from the 2016 National Survey on Drug Use and Health. NSDUH Data Review, pp. 10-11.