The study is about the effects on teenagers:

In the midst of the drumbeat toward legalization, it’s easy to forget that smoking pot isn’t great for you. Especially if you are a teenager.

A review of the research on the negative health affects of marijuana published today in the New England Journal of Medicine reports that smoking pot as a kid may have lasting impacts on intelligence and achievement.

For some adults marijuana does have positive medicinal effects, so context matters.

Also, big "correlation is not causation" here. Back in my day, the hard-core stoners were usually kids that were not gonna be on the honor roll anyway... often kids who had problems at home and very possibly a higher than normal incidence of learning disabilities like ADHD.

This study is very similar to a prohibition era one that came out some time ago. So a bunch of doctors are getting paid to rehash (no pun intended) something we all knew anyway. I think the evidence of short term memory loss is a strong enough argument without having to frame it as "worse for teens". It's bad for anybody who is trying to develop new skills, learn new things, or grow as a person.