I visualize the transition from the baby brain to the child brain to the adult brain to the older brain to be similar to vulcanization of rubber.
Rubber is very flexible in its natural state, molecular chains moving very quickly, capable of any reasonable shape configuration and consequently, very weak but with the potential for strength.
But in the process of vulcanization (which I thinks is heat + sulfur gas), the chains become cross-linked, making the rubber chains much more fixed in place but very very strong.