laws there require them to stay in school until the age of 18, and even then, they can only leave if they have a job, otherwise they have to stay until they are 21. Dutch people are keen to get good qualifications and suceed in life, and becuase they know weed can make you become a "loser", they stay away from it.
Also, the belief that Weed is legal in the Netherlands is not correct. Weed is still illegal, but the sale and use is "tolerated".
Its still a crime to smoke it in public other than in a cafe, and from my recollection, cafes are the only place it can be sold. It must also only be sold in 5g quantities and a person must never carry more than 5gs.
A Dutch person once told me that the only reason why weed is tolerated in the Netherlands is because it brings tourism.