So as I was saying, in the very first siman in Even haEzer, the mechaber writes:
It is a mitzvah for every man to marry a woman by the age of 18... and under no circumstance should he pass the age of 20 without a wife. And one who passes 20 and doesn't wish to marry, the court coerces him to marry in order to perform the mitzvah of procreation.
To this the Rema comments:
These days it is not customary to coerce regarding this matter.
And a good thing for me since at 20 I didn't know which end of a girl was up. But that's not what I'm driving at here. I'm trying to reconstruct in my mind how this radical shift occurred.
Presumably, as norms regarding marriage age shifted upwards for various economic and demographic reasons, more and more young people (and their parents) simply began pushing the envelope without giving the matter much thought. Everybody did what seemed obvious and natural and very gradually the average marriage age continued to increase. The halakhah was neither forgotten, nor struck from the books, but rather honored in the breach. Rabbanim occasionally said nu nu nu and everyone moved on. Eventually the new reality was codified.
But it might have been different. Let's imagine the following. The year is 1641 and some anal 20-year-old bachelor is determined to spend another year or two finding himself but can't abide that it says in the books that what he's doing is wrong. He is always right. So he does the only reasonable thing. He forages through the literature in search of stray opinions supporting late marriage. He marshals proofs that early marriage is a tool used by the oligarchy to oppress the masses. He commissions studies to document the deleterious effects of early marriage on men, women, Jews and the cosmos. The word is spread, societies are formed, books are written. Some rabbanim can't abide the bastardization of yiddishkeit and strike back with biting critiques of the new fashion and demand that early marriage be enforced with particular meticulousness. Some hipper rabbanim, worried that young people might be turned off, support more lenient positions as a stopgap. Breakaway institutions catering to determined bachelors and bachelorettes are duly incorporated. All hell breaks loose.
Does that sound suitably ridiculous? Has our hypothetical (and somewhat anachronistic) young friend nudged halakhah gently along or stopped it dead in its tracks? People with a modicum of foresight, wisdom, self-control and goodwill know the virtue of occasionally keeping their mouths shut. This is fast becoming a lost art.