[Dewie]

Do all kids really have such vast amounts of free time in their childhood, or is that heavily dependent on the culture? I can imagine that a lot of kids have had to work and take on responsibilities at a very young age (though I don't know if they had to do that at the so-called sponge brain age).

[barry-cotter]

Unless you've found some telepaths all children learn language by constant massive exposure. Outside of multilingual societies it'll all be in the same language too. Modern hunyer-gatherers spend ~4 hours a day doing what we'd recognise as work and this is after agricultural civilisation took all the good land. Civilisation is evolutionarily very recent. And humans have extremely long childhood period to learn probably.

Don't confuse schooling with education/learning.