This, of course, is why historically it was important that men choose younger women. The probability of the mother dying in childbirth and of the child dying in infancy was much higher, and the need for having many children to help with farm and housework was much greater. A young mother was seen as stronger and, if she survived, capable of giving birth to more children if she started younger.

Also, of course, girls were deemed as more of a burden on their father's household, and were often seen as a source of income through dowries, so they were often married off as soon as possible.

These things seem to be ingrained in society still, though we might not always be conscious of them and have (at least in western society) moved beyond them.