two big things to consider:
1. legaslation isn't going to want to pass eomthing that says an uborn fetus can smoke pot, so they will want someting similiar to the alcohol laws like 18-21
2. peoples lives(most peoples lives) start to take off around 18 or so, getting out of highschool and going to college and/or getting a job, and gaining freedom and what not, that should be the age when you can decide what you put in your body and what you don't
like i'm not sure how this would work out but as soon as you enroll in a college or get a job you can then get you cannabis licsense and smoke the herb or something like that i don't know but it's a complicated issue