Why? Mary was shocked by Gabriel’s annunciation to her that she would bear a son. Would someone who was engaged and about to be married be shocked at such an announcement? No! Childbearing is a normal part of marriage, right? Unless, this was no normal marriage - and we know that it was not. Their ages do not matter, as God chose each of them. The words of Saint Joseph, while not recorded in scripture, do not matter, as his actions spoke louder.