[trealira]

I mean, at that age, they haven't even really learned basic algebra; they still have to learn things like the order of operations, manual addition and subtraction, and manual multiplication and division.

Don't you remember reading books as a kid with child characters saying things like "I was pretty good at math class until they started introducing letters to the problems; I don't understand it at all anymore!" Things like that show that variables aren't directly intuitive to all children.