"And babies were nursed for a long time, for reasons I won’t go on about." ... But, but please?
It has a lot to do with introducing solids. Today you can puree your baby food or go to the store and buy a little jar of baby food. But the medieval mother tended to feed their babies hunks of bread softened with animal milk or water. Two problems with this.
- Bread and water/milk isn’t actually a terribly nutritional diet and so babies often stopped thriving when they were weaned.
- Animal milk wasn’t pasteurized, and therefore caused the transmission of infections.
Weaning was a very dangerous time for babies and a significant number didn’t survive it. If your kid lived past 5, their odds of reaching adulthood were pretty good, but in addition to the sizable infant mortality, the rate of death for those aged about 2 was also very high, because of these dangers. Not understanding why precisely this was the case, they couldn’t miss the fact that babies often died upon being weaned. So, they put it off as long as possible.