Aw, look at this adorable young couple frolicking on the farmland. I know they're married because I googled "happy young married couple" to find this image. Stock photographers are the new Dutch Realists, and in this image we see everything beautifully composed and heavy-handedly significant. These young folks are playing on the black soil of fertile farmland, and their bed is on the fertile farmland, and they're about to fall on the bed (which is on fertile farmland), and then the young husband will go the way of all men when they find themselves possessors of some fertile farmland, which is to plow and sow. That's what a husbandman does, is it not?
There are many commands concerning husbands in Scripture, but few specifically for or about newly married husbands. One of the few is found in Deuteronomy 24:5. This is a verse for young husbands, remarrying widowers, the occasional rich man picking up his third wife. As I said, it's a verse for when a man is newly married.
"When a man is newly married he shall not go out with the army or be liable for any other public duty. He shall be free at home one year to be happy with his wife whom he has taken."
The command is not directly to him, but to the people of God, who are to set aside as sacred the new husband's time with his bride. He is free to be happy with her for a whole year.
You know what happens when you're happy with your wife for a whole year?
What's about to happen in the picture above.
How can I put it plainly, yet in verse? Like this:
When a young man marries he may not go to war,
Is a rule that Scripture had.
For the duty he carries, 'fore he heads to the door,
Is to make himself a dad.
Y'all. One of the first duties of marriage is to make children.
I mean, you could say that this year together was simply so that a man could "properly cement his marriage", as the adorable Howard Vos did. But I just don't know how you could properly cement your marriage for a whole year in that day and age and not end up making a child.
Making children is not necessarily the highest duty of marriage. But it's one of the first. As in, chronologically. The first thing we do when we get married is go be happy with our wives.
That is to say, we don't start our career and then have kids, just as we didn't go to war and then spend the year with the wife. First comes "cement his marriage", then comes mission. Love your wife first not only in hierarchy but in time. First wife, then job. First babies, then feeding them.
First comes love, then comes marriage, then comes baby in a baby carriage. And then my career. So in answer to the question in the title, my advice is: don't wait. Have babies now. You have all your life to go to war.
P.S. I realize that the Deuteronomy verse doesn't provide an iron-clad command that all who are able ought to have kids. To those who grumpily require single-verse proofs regardless of what the whole counsel of Scripture says, to those who are married and purposefully without children, I leave the longing of the Old Testament's women alongside the difficult words of the Epistles, as I leave also the commands to God's people to have children alongside God's referring to children as blessings. Also I leave this John Piper article.