[wosk]

I did not read the paper, so I cannot comment on the "solid grasp of class inclusion", but regarding the capacity that you described in your comment, I have a 2-year-old and it's been a long while since she has mastered this (book vs this book, toy vs toy, fruit vs an apple and so on). As far as I know, most two year old have already acquired this concept.

(EDIT I see the other comment says something similar and you have replied)

[AeroNotix]

I didn't even know "class inclusion" was a thing really. Though obviously the concept makes sense.

My daughter had a solid grasp of it definitely around 16-18 months. She could easily talk about books or toys, cars, food, drinks etc.

Not sure if this is unusual but 7-8 as the other poster mentioned sounds crazy late for that kind of conceptual understanding to appear.

[dotnet00]

Yeah, it's not really convincing that it can typically take up to 7-8. By that age kids are already able to read, write and do basic math, which of course requires them to understand "classes" like numbers and letters, such that they could handle both "write a number" and "write 23".