According to this article the brain starts to decline at the age of 27 after peaking at age 22. This was the result of research done by Professor Timothy Salthouse. He carried out tests on 2000 people between the ages of 18 and 60 with the participants having to solve puzzles, remember words and spot patterns in letters. In nine out of 12 tests the average age at which the top performance was achieved was 22 with the first age at which there was a marked decline being 27. Having already reached this age, I find this research rather depressing.