That kids can learn languages quickly with not much more than consistent exposure during their critical periods is pretty clear. That adults can learn language

That kids can learn languages quickly with not much more than consistent exposure during their critical periods is pretty clear. That adults can learn languages more quickly is a different question than that of critical periods. Adults don't have that critical period to lean on, but there are efficient ways to learn languages for people who have long been outside of that critical period that can allow adults, with their own cognitive strengths, to quickly master a language to a given level of fluency at a quicker pace than children. There are entire online communities of polyglots that regularly practice these techniques and many of them didn't learn their first second language until well into adulthood.