The main reason I see a 20 year old not being ready (as opposed to girls, who are), is because we demand nothing from the boys, other than to avoid getting arrested. At 20, they are not asked to pursue a carrear, not asked to work, not asked to do anything more complicated than plan their bein hazmanim trip.
Contrast that to a girl, who at 20, will have a job, be in school so she can get a better job, and probably have a savings acct, of her own earned $.
Could we bring 20 year old boys up to speed? If we demanded it. For the moment, this conversation is placed on the back burner until bocher’l is back from EY and sprung from the freezer.
And as far as “chassidim being ready at an early age”, that is to a degree true, but if the couples I know are any indication (and I know plenty of them) their marriages hum along like a well-oiled machine because every facet (well, almost every facet) of their daily routine is micro-managed by their parents. Meals, clothing, travel logistics, ect are all managed for at least the first 3 years.
Doubtful you can find that kind of compliance in the yeshivish world. So, in all fairness to the BY girls, we really need to keep the boys off the shelf until 23-24.