Why? Because the millenial generation moves up one year in age, and as the boom parents clamp down tighter, they will do as their told. Meanwhile the reactive generation will advance in one year, making drug use slightly higher for that age bracket, but the fact is that the first cohorts of the reactives are approaching midlife where there risk taking will decline. Therefore the 16-36 age bracket will hold steady. (at least for the USA).
1997 will be the year where drug arrests among 16 year olds in the US peaks. Starting in 1998, USA drug arrests among 16 year olds will begin to decline.
For more info on age brackets - see Strauss and Howe's _Generations_, or check out book reviews at http://www.waza.com/