The phrase "immature brain" refers to a brain that is not fully developed or grown up yet. It suggests that the person's thinking and decision-making abilities are not as advanced or mature as they could be. Full definition
After all, you're parenting a very young child with a very immature brain. (attachmentparenting.org)
The relatively immature brain of the newborn -70 % of human brain growth occurs outside the womb - requires an attentiveness and care not unlike the 24 - 7 care the mother's body provides during pregnancy. (psychologytoday.com)
One such issue is that immature brain tissue may hemorrhage. (babygaga.com)