Years ago there was this Channel 4 documentary called 'Extraordinary Breastfeeding' and I was a teen when it was on and I watched it with my parents (we only had 2 TVs in the house). The documentary used to be up on YouTube in various mirror uploads but it may be gone now.
There was one Australian couple in it who were living in the UK who were breastfeeding the youngest kid at 8 years old (on-demand but it was only in addition to the rest of her food and occasionally) and there was an older kid who was upset at being weaned yet the younger sister was still allowed a bit of milk.
On the TV show, the voice-over said 4 is the average age of weaning worldwide and at the time, I assumed maybe the benefits of breastmilk are useful until that stage but it is possible it's easier for mothers in third-world countries to supplement whatever meagre food they have with their breastmilk but it's not a baby-health essential to drink some milk until you are four.
The 8 year on the show was quite overweight for her age and height which suggests breastmilk just packs on excess calories and fat when you're eating normal foods. She stopped feeding in the program because her tongue was maturing and unable to suck a nipple but the girl was insisting she could still suck milk anyway.
There was also a mother feeding her 2-year-old twins and the weaning was shown in the program and because it stopped cold-turkey from on-demand as opposed to schedule milk, the kids got really upset in the show. My father did say when we were watching it because they were 2-years-old, nutritionally there was nothing they got from the milk that they couldn't get from ordinary food.
I found one clip of the Aussie family