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Teen marijuana use held steady in 2014, the first year that marijuana was legally available for purchase in the states of Washington and Colorado, according to just-released numbers from the Substance Abuse and Mental Health Services Administration.

Many opponents of legalization have warned that legal weed would lead to a spike in the number of teenagers using and abusing the substance. New Jersey Governor Chris Christie has frequently said that legalization would "send the wrong message" to kids. Federal drug czars have often echoed a similar refrain.

But these numbers suggest that so far, national conversations about marijuana legalization haven't led to an increase in teen use. This comports with the best available research on marijuana laws, which finds that marijuana laws have have so far had little impact on overall teen use trends.

Nationally, 7.4 percent of kids age 12 to 17 use marijuana monthly, according to SAMHSA's new numbers. That's up by 0.3 percentage points from the prior year, a change that SAMHSA says is not statistically significant, but still significantly lower than levels seen in the early 2000s. There has, however, been a significant increase in the number of adults age 26+ smoking weed monthly, from 5.6 percent in 2013 to 6.6 percent in 2014.

The implication is that marijuana legalization in Colorado and Washington has had little to no effect on national teen marijuana use trends, but perhaps has contributed to the increase seen in adult marijuana use. SAMHSA will release state-level numbers later this year, which will give a clearer picture of what's happening in Colorado and Washington.

If you're an advocate of loosening marijuana laws , this is exactly what you want to see -- more adults using a substance that's now legally available to them, with little impact on teen use.

Of course it's early, still, in the national legalization debate. We won't really know the full effects of changing marijuana laws until four or five years down the line, when markets have matured and states have had time to track data on use and abuse.

But so far, there's been little apparent downside to legal weed in Colorado and Washington. Colorado marijuana taxes are bringing in millions of dollars for schools (ditto in Washington). Cops are spending less time arresting and jailing marijuana users. Crime and impaired driving in Colorado is generally flat or trending downward.

Anti-marijuana groups keep warning us that the sky's about to fall, and that each incremental shift in drug policy will lead, inevitably, to catastrophe. But the actual data continue to show otherwise.