correlation does not mean causation
this
first, generally less intelligent people are more likely to use drugs (for serious ones, because they're not so dumb as to warrant the side effects; for stuff like marijuana, maybe they're just fine relieving stress on their own)
second, yeah of course people who smoke at a young age are going to have issues but as Swholli said there's an age restriction
alcohol doesn't permanently lower IQ in adults if dranken at normal levels, but if it's a 7-year-old of course it will