Does the brain really stop developing at 25?

No. The brain keeps changing well into adulthood, but there is no strong evidence of a sharp cut-off at 25. The "brain matures at 25" idea is a widespread myth: the belief that the human brain reaches adult-level maturity at around that age.

It grew from real findings. Jay Giedd's study found in 1999 that frontal gray matter peaks in pre-adolescence and then thins through synaptic pruning, with the prefrontal cortex among the last areas to mature. Later, in a Time article, Giedd speculated that when they started they thought they would follow kids until about 18 or 20, but if they had to pick a number now it would probably be 25. Laurence Steinberg, who has himself proposed that 10- to 25-year-olds have immature brains, said in 2022 that he was not sure where the "25" number came from.

What does change in the teenage brain? White matter increases while grey matter follows an inverted-U pattern as unnecessary connections are pruned, and losing grey matter does not mean losing function: the brain becomes more efficient through myelination and fewer unused pathways. Casey's imbalance model proposes that these changes create transient imbalances in how brain areas communicate, affecting emotion reactivity and regulation.

The myth matters because it has been used in legal and policy circles to argue that people 25 and under are neurologically immature and less culpable. But research indicates that adolescents and adults think about risk in similar ways while holding different values, and that adolescents give more weight to rewards, particularly social ones.