The “Show me ONE psak” is a straw man. Many commenters above pointed out that various Yeshivas have guidelines, a.k.a. a psak, regarding a preferable marriage age and none of them are at 18 or 19. The demand for one psak is laughable, does a psak halachah have legitimacy only after it’s published in a book? How absurd.
The Chazon Ish not only married “late”, he also married a woman who could no longer have children. I dare say that in itself is a psak. The requirement to marry at 18 or 19 is simply not halachah, it is a recommendation and is subject to the times. That’s why we still have poskim, whether they live in America or Israel. It is obvious that the modern day poskim have paskened, whether they printed it or not. Boy or girl, get married when you are mature enough for the commitment and find the right one.