> Mathematical maturity has all to do with practice and experience and nothing to do with age. That's OP's point. "Age-appropriate" was put in quotes because

> Mathematical maturity has all to do with practice and experience and nothing to do with age. That's OP's point. "Age-appropriate" was put in quotes because it was referring to a metaphorical "mathematical age" to tie it to the concept of mathematical maturity. And, indeed, certain books require more mathematical maturity to get through than others, even though the prerequisites may be minimal. You'll see this often explicitly described in the preface of textbooks and reviews of those textbooks. They mentioned studying certain books to develop mathematical maturity because that's where the practice and experience happen. Calculus is one such course used as part of this process, as are many courses intended as a first exposure to proofs like linear algebra and discrete math. Some might use the Moore Method with point-set topology.