The only thing that worries me about legalising it, is that it takes the act of "doing something that they shouldn't" away from the younger generation, and may take a bit of the appeal away from them, and cause them to go onto something harder that is illegal.
Well, I don't know about other countries, but I'm sure here it would still be illegal for teenagers to smoke pot, because it's illegal for them to drink. The fact that it's legal for adults to drink does not seem to take away the appeal of drinking for teens.
In fact, you might as easily argue that it would keep them from going into something harder. Lots of teenagers drink alcohol but don't do drugs. Maybe those who smoke pot are that much more tempted to try harsher drugs because they've already broken the law in a big way by using an illegal drug, they've already established markets for buying and selling illegal drugs.
It's handy to be able to tell my sons they can't smoke pot because it's illegal. But they also can't do a lot of other things that aren't illegal because they're kids. They can't drink, they can't get married or enter the military, they can't quit school, they can't buy a house. So if marijuana were legalized, the rule would become one based on their legal status as juveniles rather than the legal status of the substance.