@Yogi DMT,
First of all, please don't believe every song line you hear, nor movie plot you see, nor passage you read--without firstly checking it out some. (which is of course what you are doing here . . . a very commendable thing, indeed !!)
As far as can be understood at the moment, and with the last papers I have read on the subject, the likelihood is still very high that the last brain area to recieve full myelination is the prefrontal cortex--which is the major behavioral guide/inhibitor/association/cognitive structure. This occurs generally in the late teens to early twenties.
What this means, however, is simply that after mid-adolescent pruning (automatic cell death occurring to 'streamline' what is/isn't needed) the speeds and accuracy of axonal signaling is increased. It is understood that judgment is improved after full myelination has occurred (in most cases, to varying degrees), and this does seem to be the case in observation. Otherwise, any pursuits which one may have due to the words of that song, will surely come to nothing.