However, there was some decline in their psycho-motor speed. This means that it took them longer to accomplish mental tasks than it used to. But when speed was not a factor, they lost very little intellectual ability over the years. In general, Dr. Jarvik’s studies have shown that there is no decline in knowledge or reasoning ability. This is true not only into the 30s and 40s, but into the 60s and 70s as well.