ויאמר אלקים פרו ורבו
בראשית א – כח
To have children
So that the world should be settled and so there should be people to perform all the Mitzvos
Anytime, but the requirement is only from when he is suitable
In Yevamos Perek 6 and Brachos page 16
If you don't fulfill it
He has neglected to perform a Mitzva. He gets a big punishment for he has shown that he doesn't care to fulfill Hashems will to settle the land.