Many thanks.
If you are not yet 18 you will be asked to leave the coffee shop, and fast, as when they are checked and you are on the premises they will get a fine and can get closed.
Everybody has to carry as passport (or other acceptable ID, for European tourists it can be their official ID if they have one, for the rest it has to be a passport.)
So they will not accept your 'forgot my passport'. No passport no weed.
Smoking weed/owning weed is not legal in the Netherlands, it is just not acted upon when you are not a problem in other ways, with a minimum user age of 18, and not a day less.
Most coffee shops won't even let you in without ID. I went to three different coffee shops with a group of people age 37-43 and we all has to show IDs before we could go in. At night there is usually a security outside checking IDs. If you go in the day they will check your ID when you order and make you leave if you don't have ID
Chances are you won't set foot in the shop. But if you do get in one of the smaller ones you will be refused and laughed at and asked to leavee
the rules in amsterdam are the same as in the rest of the netherlands except the three southern provinces. In those they don't serve anyone who doesn't live in the netherlands
They'd just refuse to sell it to you.