You DO have to be 18 to smoke in coffeeshops in Amsterdam.
I got ID quite a lot when I was there a few weeks ago...

But you have nothing to lose, its just like trying to get into a club....the worst that could happen is they askfor your ID ....and then say "no sorrywe cant serve you"
Im sure if you have facial hair atleast one coffeeshop will serve you just try around and act relaxed...

Worst comes to the worst you get to spend 20 hours in the beautiful city of Amsterdam haha

Give it a go though, there is no harm in trying

(I dont know about laving the airport....I dont see how they could make you stay there)