Follow-up question: Would this be similar across cultures? I don't expect answers for each country, but would this idea be similar in the US, as in Europe, in Asia, in the Middle East, etc. or would it be radically different?

I'm an early modernist, so I can speak to that period but not really before.

Sure, plenty of people in a variety of societies have gotten married at 13, right around the age of majority/menstruation, but most waited longer. And in many of those cases, the cases that we know about, we are mostly talking about the nobles, the 1% of society. They are betrothing their children at a much lower age than the common person.

The average age at marriage for an early modern Frenchman? Around 28. And 25 for women, which is not that far off from the modern day (and that three year gap has held more or less consistent as well). This is because it was expected in western Europe that a new couple be able to establish a household and a business/farm immediately upon marriage. And part of the reason they would wait is because if you are in a town with a largely static population (because the local region has reached its carrying capacity) then you can't really start a household until you inherit a farm from a parent.

This was a bit different in southern Europe, but even then marriages would usually occur in the late teens, and the couple would continue to live with one of the families until they were prepared to move out on their own.

Are they waiting until marriage? Sometimes, but more often than not marriage waits for them. It was common practice when a girl got pregnant to say that it happened "because the man had promised marriage" whether that was true or not. That saves face for all parties and ensures that the pregnant girl and her child will be taken care of. Yes, shotgun marriages existed before shotguns did. And there were plenty of illegitimate children outside of this.

And another note, brothels were largely legal in early modern Europe (less so after the Reformation) in part because it was understood that brothels were a method of protecting the "purity" of the rest of the women. As we all already knew, there is a gender component to "no sex before marriage" in that if the man has sex, it's pretty much okay, as long as no one ends up pregnant.

My point is that even if you have a rule like "no sex before marriage," which again, was most important for the nobility (who had property to inherit), not everyone is going to follow it. Even in the USA, which is ostensibly 70% Christian (and a fair percentage of other faiths who feel the same way about fornication) the vast majority of people have sex before marriage. On the one hand, people in the past were very different from us, but on the other...they were still horny youths, and had little access to birth control. So we can see how often they broke that rule in marriage/birth records.