text: The mean age at test is 32 and the mean year of birth is 1977, implying that most of these 
  individuals were in their mid-to-late teens during the crime peak of the mid-1990s.9 Given 
  evidence suggesting that the average age of initiation for opiate/crack use is around 18–20 
  (Millar et al., 2001), this age profile would tentatively suggest that OCU incidence also peaked in 
  the 1990s and that this created a large cohort of users who would be approaching 40 today. 
  
  The minimum and maximum years of birth are fixed by construction, because anyone born 