Sorry to bring you the bad news, guys. Timothy Salthouse, Professor of Psychology at the University of Virginia studying cognitive aging, found that reasoning, speed of thought and spatial visualization start to decline at age 27:
The first age at which there was any marked decline was at 27 in tests of brain speed, reasoning and visual puzzle-solving ability.
Things like memory stayed intact until the age of 37, on average, while abilities based on accumulated knowledge, such as performance on tests of vocabulary or general information, increased until the age of 60.
Professor Salthouse said his findings suggested "some aspects of age-related cognitive decline begin in healthy, educated adults when they are in their 20s and 30s."