You don't *have to* legalize it to provide grants for study (obviously, as some studies have already been done!). Increase the funding for study. One or two studies showing permanent neural network changes after light, transient use in adolescent brains does not suggest (to me at least) that light, transient use *always* results in neural network changes. But if, after additional study, this turns out to be the case, we need, I think, to understand the implications of those changes to the neural networks as those affected adolescents mature. If there is no measurable negative effect, then maybe you take the chance of setting the age limit at 21 (although that's a scary proposition, imo). The studies I've mentioned about this before noted, significantly, that no permanent neural network changes occurring from light, transient use were observed when the use came after the full maturation of the brain (this would suggest that a reasonable age for legalization would be 25 - I'm not advocating that, there, again, hasn't been enough study). I wouldn't base any decision solely on the one or two studies that have demonstrated these results. I would want more compelling evidence and a more thorough understanding of what is causing neural network changes, if they are indeed lifelong, if only adolescent brains are affected and what the long term effects on mental functioning are (if any) among individuals whose neural networks have been affected.
Once these things are known (or at least better understood) then an adult decision can be made about age limits for use under the law.
Once these things are known (or at least better understood) then an adult decision can be made about age limits for use under the law.