Well, and if you're not trying to imply that we are at peak cognitive ability at the age of 13 (and I at least hope you don't believe that) this should be hint number one for you that the way you think maturity works is probably wrong.

It also shows me that you didn't even read the short excerpts I provided for you. Here, again:

Searching for Signatures of Brain Maturity: What Are We Searching For?

(...) For the current discussion, the key point is that there is no single progression that encompasses functional maturation. Neural activity intensifies and reduces, varies quantitatively and qualitatively, in linear and nonlinear ways that are both linked to—and independent of—behavioral differences across development. Each of these patterns reflects developmental progress, but the wide range of ‘‘journeys’’ prohibits a simple definition of what emerging brain functional maturity looks like. (...)

Or another quote from that paper:

(...) Measures of widespread brain connectivity shift in complex ways from childhood to adulthood, characterized by reductions in local connections and rises in distributed connections. These connectivity-based shifts are thought to reflect a brain that is becoming more efficient in its in-network communication and more integrated in its cross-network communication (...)

Since you seem confident in your grasp of the topic I guess those two should answer your question.