The study was of 104 chronic users; 49 of those started toking before 15. The average length of use for the pre-15 group was over 10 years. The article does not say how long the other group smoked, but if they were of the same age at the time of the study, then the earlier group would have been smoking longer. I don't think they were.
Surely long term use screws up the brain, but the conclusion that teenage use is more destructive than long-term use doesn't follow from the data provided in the article.
Who was the long-term toker, the journalist or the scientists?
No amount of evidence will ever convince Woodstock libertarians. Review some brain scans if you’d like more proof.