“Delaying any chiuv is a very serious matter and one needs a very good reason to do so, even more so with the Torah’s very first chiuv. Further, in the Mishnah, the age for marriage is 18. ”
I strongly disagree on all three points.
One: The fact that pru u’rvu is the first mitzvah does not lend it any increased importance. I shudder at the implied suggestion that the mitzvot are mentioned in the Torah in order of their importance.
Two: Many mitzvot are delayed and it is not a “very serious matter”. Otherwise, one would marry at the bar-mitzvah. Other examples? Do we require the bar-mitzvah boy to immediately write a Sefer Torah?
Another example: there is no obligation to separate trumot u’ma’asrot immediately after gathering the produce…and many more
Three: If one reads the entire mishnah in Avot, it is obvious that these age-related statements are not iron-clad. I know of no yeshiva that abides by ben chamesh l’mikra etc…, and to think that ‘ben arbaim l’binah’ is an obligation is a good joke. Furthermore, a glance at the poskim reveals that the marriageable age varies from period to period and comes with caveats.
One should get married when he can fulfill the requirements of the kesuvah they read under his nose as he stands in anguish under the chupah.