Age-related Changes in Past-Month Alcohol, Cannabis, and Simultaneous Use in a Statewide Sample of Young Adults in Washington State.

Objective: It is unknown whether age-related decreases in substance use (maturing out) are observed in the legalized cannabis context. This study evaluated age-related changes in past-month alcohol use frequency, cannabis use frequency, and any simultaneous alcohol and marijuana/cannabis (SAM) use among young adults who engaged in the respective substance use behavior.

Methods: Young adults, residing in Washington State at enrollment (N=6,509; 68.3% female; ages 18-25), provided 3-5 years of annual data in a longitudinal, cohort-sequential design from 2015 to 2019, a period after nonmedical cannabis was legalized and implemented. Multilevel growth models were conducted; post-stratification weights were applied to make the sample more similar to the Washington young adult general population in demographic characteristics.

Results: Among those who reported alcohol use at 1+ timepoints, days of alcohol use increased from age 18 to approximately age 25 and then decreased until age 30. Among those who reported cannabis use at 1+ timepoints, days of cannabis use increased from age 18 until approximately age 23 and then decreased until age 30. Among those who reported SAM use at 1+ timepoints, the probability of SAM use increased from age 18 until approximately age 24 and then decreased until age 30. Age-related changes in SAM use were largely explained by concurrent changes in alcohol and cannabis use frequency.

Conclusions: Maturing out was observed for alcohol, cannabis, and SAM use among those who used each respective substance, with evidence that age-related changes in SAM use were tied to alcohol and cannabis use frequency.