Counting and arithmetic are themselves abstractions. The concrete reality is "this is a raspberry. this is another raspberry" etc, and that's about where my kid was at 2 (his loveys are named "Owl" and "Another owl"). Then we learn that there can be 5 raspberries just like there might be 5 ducks or 5 bricks, and that taking away 1 raspberry is like taking away 1 duck or 1 brick.
I could believe that the ability to abstract develops at different times in different kids. But I'd caution about taking Western research and generalizing it across all cultures, or generally assuming that kids are less adaptable than they are. There are existence proofs for kids learning algebra at much younger ages than most American schools do (3rd and 4th graders do in eastern Europe, as do Americans who participate in RSM).
I could believe that the ability to abstract develops at different times in different kids. But I'd caution about taking Western research and generalizing it across all cultures, or generally assuming that kids are less adaptable than they are. There are existence proofs for kids learning algebra at much younger ages than most American schools do (3rd and 4th graders do in eastern Europe, as do Americans who participate in RSM).