Scott Alexander at Astral Star Codex, as usual, is the public intellectual who decides to look at whether this rather universally-held idea that the brain does not fully mature until age 25 is in fact true. As Alexander often does when developing his case, he tries rather obviously to force himself not to draw premature conclusions. But you can tell quickly it has occurred to him that this idea is a myth and he wonders whether he will be proved out. I appreciate the effort he makes, telling himself not so fast.
I believe we like the idea because it accords with what we see, or think we do. A boy will run up and leap over a fence if his brain gives the assessment "I can make it nine times out of ten. It'll be cool." After a few falls and failures that slides back to "I can make it 99 times out of 100" and eventually 999 times out of 1000, which we call better judgement. Eventually one gets to the point of "Wait. Why am I jumping over fences at all, unless there is some need (or perhaps for training, or to test oneself for an upcoming trek across terrain. Better to learn that here where the ground is softer and I'm closer to the car.)?"
But the data Alexander references shows that the brain parts of development are complete at about age 15 and then steadily decline throughout the lifespan, with no identifiable peaks or breaks at 25. As we don't think of 15-year-olds as showing the best judgement among us, that seems odd. We take fewer risks from that point on at a steady pace, as with jumping over fences. We do not hit some optimum of survival-based judgement at age 25 and then hold that until senility. At least, our brains don't.
Let me propose that our perceptions are guided by something else: the optimal level of risk-taking in our current culture occurs a few more years after the end of puberty than it did fifty or a hundred years ago, and as complicated and technological civilization has developed, the number has been slowly rising for a long time. For the bulk of human existence, the amount of risk-taking we show at 15 has been about the right amount for perpetuating ourselves. If that's the case, then getting them "safely" married off by that time is a good strategy, fully acknowledging their sexual impulsivity while also insisting on an awareness that they must "use their Wise Minds," as the DBT phrase goes, rather than just do whatever comes into their...uh, heads.
It's a fun article to contemplate, as mythbuster articles by objective people usually are.