According to the report
Drug Trends in the Seattle-King County Area, 2008, of all King County residents who entered substance abuse treatment in 2008, 81% of those who were admitted for alcohol addiction started using alcohol before the age of 18. Ninety-two percent of those who were admitted for marijuana addiction started using marijuana before the age of 18.
The chart below shows youth substance abuse treatment admissions in King County. By far, the primary drug for which youth seek treatment is marijuana, followed by alcohol.