If the trend will be for boys to marry at 20-21 they will begin thinking about the future at a younger age (say 17-18), the Rabbeim will begin speaking about building a home at a younger age thus making them more mature, thereby making boys more ready for marriage at a younger age.
You can’t tell a boy to get married at 20 to fix the system. But, if he knows that he will start dating at 20 chances are he will “settle down” in thought and in spirit at 18, and then by 20 he will be Be’zras Hashem be ready.