>After infancy, the rate of myelination slowed during childhood and adolescence, but exhibited continued growth until the end of the third decade. Specifically, mean MFLD during later adulthood (defined as ≥28 y) was significantly greater than in adolescence and early adulthood (11–23 y; Welch t, P = 0.000059; Mann–Whitney U, P = 0.00016).
https://www.pnas.org/content/109/41/16480
Our (dorsolateral) prefrontal cortex is one of the last parts to myelinate.
>The last areas to myelinate are the anterior cingulate cortex (F#43), the inferior temporal cortex (F#44) and the dorsolateral prefrontal cortex (F#45).
https://en.wikipedia.org/wiki/Myelinogenesis
This is the area of the brain that handles things like planning and abstract reasoning (though it isn't the only area involved in such thinking).
https://www.urmc.rochester.edu/encyclopedia/content.aspx?Con...