הלכה על אבות 5:21

And [it] is practiced in every place and at all times (Kiddushin 36b), and a person is obligated to be involved with it from when it is fitting for him, and this is the age which the sages (Avot 5:21) designated to marry a woman. And this commandment is not incumbent upon women. And one who negates it violates a positive commandment; and his punishment is very great (Kiddushin 29b), as he shows that he does not want to fulfill God's will to settle His world.