One thing that isn't sitting well with me with respect to the article is the way they reference 'declining' IQ.
The information they give seems much more to say that pot smokers' IQ < the rest of the population, but that is not technically a decline.
A decline would be an IQ of 100 at age 18 and an IQ of 92 20 years later. While they did reference that example, it always seems to be in the pot smoker vs population manner, not pot smoker vs themselves.
This has a significant impact on determining causation.
For example, I'd wager money (I'll even give you odds Pokerguy) that WWE watchers have a lower IQ than the rest of the population, and that people who continued to watch it into their adult life had lower average IQ at 38 than the rest of the population. That would not show that watching WWE lowers IQ though.
Please note that I didn't intend to offend fans of Vince McMahon's wildly successful soap opera.
This is an excellent point.