Before drawing conclusions, though, let's run through the study and its implications.
The study: Data from 3,725 students from Australia and New Zealand was analyzed to investigate "association between the maximum frequency of cannabis use before age 17 years (never, less than monthly, monthly or more, weekly or more, or daily) and seven developmental outcomes assessed up to age 30 years (high school completion, attainment of university degree, cannabis dependence, use of other illicit drugs, suicide attempt, depression and welfare dependence)." The researchers claim they controlled for other factors such as age, sex, ethnicity, socioeconomic status, use of other drugs and mental illness.
The researchers found "clear and consistent associations" between the high quantities of marijuana young people reported ingesting and "all adverse young adult outcomes." So pretty much every one of the seven developmental outcomes was tied in some way to marijuana use before the age of 17, and the problems got worse as teenagers consumed more and more marijuana.
But wait a second: This looks pretty bad, but the study's implications for the liberalization of marijuana laws aren't quite so cut and dry. The Washington Post's Christopher Ingraham runs down a couple of confounding factors, including that drug-using teens are often stereotyped by teachers and have run-ins with the law that those who use alcohol may not, which can have an adverse impact on their educational outcomes. They may also be falling in with the wrong crowd, like drug dealers, which means that many of the problems teens are running into might actually have to do with the stigma and legal penalties of regularly using an illegal drug, rather than marijuana itself.
Moreover, "Won't somebody think of the children?" might have some scientific backing in this case, but the results just mean state and local governments need to do a better job of keeping marijuana out of the hands of teenagers, something that hasn't necessarily been accomplished by decades of prohibition. Despite abrupt shifts in attitudes towards marijuana, CDC shows marijuana use among high schoolers hasn't grown at all and in fact remains lower than it was in 2001. Another study found no evidence that states with legal medical marijuana, like Colorado, were experiencing higher rates of teen use.
Finally, teen drinking is associated with a countless array of other destructive effects, but few people have suggested bringing back the alcohol prohibition of the early 20th century. Marijuana has a variety of medical purposes and can be used recreationally by responsible adults, and there's no evidence whatsoever that a well-regulated legal environment for weed would dramatically increase teen use.
Habitual marijuana might not be healthy, but that doesn't mean the government should continue an incredibly harmful war on drugs that uses heavily armed police and the threat of criminal penalties to keep everyone away from marijuana. It's not working, which should be reason enough to consider alternatives.