"Cortical white matter increases from childhood (~9 years) to adolescence (~14 years)," while "cortical grey matter development peaks at ~12 years of age in the frontal and parietal cortices, and 14–16 years in the temporal lobes" [1].
The latter processes emotions and language [2]. Its myelination continues significantly through at least 17 years old [3], through one's mid twenties.
[1] https://en.wikipedia.org/wiki/Human_brain_development_timeli...
[2] https://my.clevelandclinic.org/health/body/16799-temporal-lo...
[3] https://pubmed.ncbi.nlm.nih.gov/33359342/#&gid=article-figur...