I don't think that a developing brain (13 years old) should be smoking cigarettes, weed, or drinking alcohol. They have found that a person with a un-developing brain who smokes weed compared to someone who doesn't during that same time period can lose up to 8 iq points like you said, but the majority didn't have this issue. But they have also found a lot of positive things that pot does (Recent studies show it can help low blood sugar)