If you are <18 and look it, then its like alcohol and you wont get served, despite the easy going nature of the Dutch. Also, they do not want to get into any legal issues and screw it up for everyone, so dont push the issue if on doubt. As the others have suggested, go and find someone.
you can go into a coffee shop; some paretns take their kids in there as the atmosphere is nicer then bars and stuff, and you should be able to find a like minded soul who'll go buy some stuff for you.
If you are not used to it, go for something low potency first....Amsterdam gear is in a class of its own.And if you are there next week, I'll see you there. I'll be the chap wearinga duffle coat and a large fur hat.....its ging to be f***ing freezing!