I agree that less time and more obligations does make learning new things more difficult later in life. But there's also evidence that fluid intelligence [1] peaks in young adulthood and declines with older age.

[1] http://en.wikipedia.org/wiki/Fluid_and_crystallized_intellig... : "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."