That also happens through development, and in fact, it goes through development and even teenagers are not fully myelinated.

髓鞘的形成也同样发生在发展过程之中,实际上,髓鞘的形成贯穿整个发展过程,即便是青少年的髓鞘也并未完全发育完善

耶鲁公开课 - 心理学导论课程节选

In particular, they're not fully myelinated in their frontal lobes.

特别是他们额叶的髓鞘并未完全发育

耶鲁公开课 - 心理学导论课程节选