In reply to Kevin Viel.

Good question. The chart shows the percent of respondents in each category who say they used marijuana in the prior 12 months. The actual number of respondents for each category (yes or no) is available only in some of the report data. At the national level, I can get the unweighted and weighted counts for each of these 3 age groups. Here's the counts for 2017:

AGE WEIGHTED UNWEIGHTED
12-17 Years Old 24,942,794 13,722
18-25 Years Old 34,306,312 13,840
26 or Older 212,854,229 28,714

So yes, many more actual respondents in the '26 and older' age group, but even more interesting is how the sample size of that group compares to the size of the weighted population.