Among youths aged 12 to 17 who have ever tried marijuana, the mean age of initiation is 13 and a half. The mean age of initiation among adults aged 18 to 25 who have ever tried marijuana is 16.
But of course, as Murray notes, that proves precisely nothing:
It should come as no surprise that the mean age of first use for a younger group is less than that for an older group. What about all those in the 12-17 group who haven't tried marijuana yet but will do when aged 18 or older? This is a meaningless finding, yet it is advanced seemingly in support of CASA President James A Califano's assertions about the dangers of the drug.