But outside the home, there is NO WAY to stop minors from getting it if they really want. It is a frickin weed. When I was about 14-15, I knew plenty of kids that grew it in the woods and then sold to classmates. That was up north but in southern climates kids have almost unlimited quantities.
Unless you have 24/7 surveillance of everyone everywhere, you will never put a dent in pot consumption. It is easier to grow than tomatoes and grows in every corner of the globe except the poles.
Then why set an age on pot or alcohol?
Don't let them get driver's licenses until 21.