You weren't "young and stupid." At 21 years of age, you were a stupid adult. Don't try to minimize the choice you made.
You'll have to ask the department you want to get hired on with. Each one may have different standards than the rest. Those standards MAY change in the next 2 years (becoming stricter) as you're taking classes or they may not. Even they can't predict the future. They'll want to know LOTS more information about your drug history. You didn't smoke marijuana 1 single time to get it in your hair sample.
Of every one hundred men, ten should not even be here. Eighty are nothing but targets. Nine are real fighters, and we are lucky to have them, for they make the battle. Ah, but the One... One of them is a Warrior... He will bring the others back.
"Wrong door, buddy!"
Let no man's ghost say my training failed him.