Brain Decline Begins At 27


A 7 year study, conducted by Professor Timothy Salthouse of the University of Virginia, found that 27 was the first age at which there was a marked decline in "brain speed, reasoning and visual puzzle-solving ability."

Read the article here.

His study has been published in the journal Neurobiology of Aging. If you're interested in the full study you should be able to get it using the following info:

Title: When does age-related cognitive decline begin?
Author: Timothy A. Salthouse
Volume 30, Issue 4, Pages 507-514 (April 2009)