False, it was questioned. Even Nature, the world's most selective journal, which consists of some of the brightest minds in science in its committee, said further study needed to be done to completely debunk the lower intelligence study. In fact, if I remember correctly, they even admitted that, due to previous studies even other than the New Zealand one, smoking cannabis WAS correlated to lower working memory and intelligence. Saying the previous studies are completely debunked is as far away as you can get from the truth, all that went down is many scientists said "more studies need to be done'. That's called questioning a hypothesis/experimental results, and is NOT synonymous with debunking.

Then don't use it before 18, like they tell you to do with alcohol and tobacco...