From the Wikipedia, which has references: 'Fluid intelligence, like reaction time, typically peaks in young adulthood and then steadily declines. This decline

From the Wikipedia, which has references: 'Fluid intelligence, like reaction time, typically peaks in young adulthood and then steadily declines. This decline may be related to local atrophy of the brain in the right cerebellum. Other researchers have suggested that a lack of practice, along with age-related changes in the brain may contribute to the decline.'