Age of Litvish Godol at his marriage:
Chazon Ish;27
Brisker Rov : 24
Rav Aharon Kotler: 22
Steipler Gaon: 28
Rav Schach: 26
Rav Moshe Feinstein: 27
Rav Chaim Kaniefsky: 24
I think this list quite speaks for itself regarding the obligation to marry at age 18-20