Technically the brain never finishes developing due to neuroplasticity. 25 tends to be the cutting off point for frontal lobe growth, but it isn't like a cut and dry thing. Some people have it develop much earlier, and others have limited growth due to various factors. I feel like it would be ableist to base adulthood on a stereotypical 'developed' brain structure as it would remove autonomy for pretty much every neurodivergent person.