The maharaha writes that for learning Torah, one is allowed to push off getting married until 24, and that was the minhag of the Litvishe yeshivos in Europe.
Let’s say someone is 20, but has no job, no ability to sit and learn, and has the middos of a sailor. Is he really ready to get married? If someone cannot marry at the “perek”, the normal time, then he is an oness. If someone does not have the ability, whether in terma of maturity, emotional stability, financial ability, or any number of considerations, then he is simply an oness.
It would take a gadol beyisroel to say that “we’re all onsim” or something to that effect, but I happen to know several choshuve rabbonim whose children got married very young because they were working, and the aforementioned maharsha didn’t apply to them.