High school. According to that website, the effects depend on how it's ingested and how much. It also said it runs $50-100 per ounce. I only know of 2 kids who have so much as talked about salvia, my best friend and another guy in my chemistry class, although I'm sure it has more recognition, I'm just not that popular or into drugs.
Since I haven't ever smoked, I'm going to venture a guess that pot is more widely available, hell my friend's dad grows the stuff so there you are.