With legalization comes acceptance. With acceptance comes more use by respectable grownups.
About 9 percent of U.S. adults between ages 50 and 64 used marijuana in the the previous year, according to survey results. About 3 percent of people over 65 used the drug in that time period. This appears to be up from years past. In 2013, the same survey reported that 7 percent of middle-aged Americans used marijuana in the previous year, and only 1.4 percent of people over 65.
Here's the research: Marijuana use by middle-aged and older adults in the United States, 2015–2016.