activate - you say "Because babies used to have to make way for the next ones along and it would not be normal for a 4 or 5 year old to be breastfeeding when they have a 3, 2, 1 year old sibling "
This is of course true in some cultures, some of the time.
But even now, especially in pre-industrial societies, it is absolutely usual for a child of this age to continue breastfeeding, sometimes alongside a younger sibling. In some societies, the older one is fully weaned as soon as the next pregnancy is known about. In other societies, this is not the case.
It's daft to talk about 'the past' as if it was all the same.
What we know, from current observations, from paintings and pictures from the past, from archeology and knowledge of ancient and not-so-ancient cultures, is that it is normal for children to continue to breastfeed for longer than the current 21st century western expectation (at a guess, some time around 6 mths of age) - breastfeeding to 4, 5 and 6 is not unusual at all if you take a historical and anthropological view :)