Cognitive decline may start as early as age 27

Telegraph - Old age begins at 27 as mental powers decline, scientists find

In a 7-year study conducted on 2000 18-60yr-olds, noticeable levels of declining performance on some cognitive tests appeared around 27. Decline in cognitive speed, reasoning tests, and spatial visualization appeared around 27. Memory decline appeared around 37. Performance on vocabulary or accumulated general knowledge increased until around age 60. The peak performance on many of the reasoning tests was 22.