"Daily marijuana use held steady at 6.8% in 2015 among young adults, but that is triple the rate in 1992 (2.3%), the low point since estimates for this age group first became available in 1986. The 2014 and 2015 rates are the highest levels of daily use ever observed in this young adult population since tracking of their use began 29 years ago."

Click here for complete datatable of Thirty-Day Prevalence of Daily Marijuana, Alcohol, and Tobacco Use Among 19-30 Year Olds in the US

Source

Johnston, L. D., O’Malley, P. M., Bachman, J. G., Schulenberg, J. E. & Miech, R. A. (2016). Monitoring the Future national survey results on drug use, 1975-2015: Volume 2, College students and adults ages 19–55. Ann Arbor: Institute for Social Research, The University of Michigan, p. 141, and Table 4-5, p. 117. Available at http://monitoringthefuture.or…
http://monitoringthefuture.or…