I think 16 is a decent year to start.
I knew a 16 yr old stoner and he was coo.
I met this 14 yr old stoner who looked way too young to blaze and I kinda felt sorry for him..
But my sister first started at 14-15 and I don't mind.
That 70's Show gang started when they were 14-15 in the episode where Red asked when they started smoking then Eric and Hyde day dream back when they first did it.

Though I remember my old friend back when we were 12-13 who used to smoke a lot of weed.. and cigarettes. We all thought it was a bit weird but we always accepted him cause he was really smart and cool.

I started in the beginning of 12th grade. I guess it was late, just didn't feel ready yet.