It’s not just a matter of maturity; it’s a matter of willingness and capability of assuming real responsibilities, and knowing that once the chuppah’s passed, there may not be the same opportunity to have such intense, single-minded learning. A 23 y.o. could handle that; for most 20 y.o.’s this is really unfair to them.
And should we assume that if it becomes standard for boys to marry younger, that the ante will be upped as far as expected years of support?