Scientists at the University of California Los Angeles are reporting that while some people may think "life begins at 40," all it seems to do is slow down. According to recent research, at age 39 our brain reaches its peak speed, and it's all downhill after that. "The loss of a fatty skin that coats the nerve cells, called neurons, during middle age causes the slowdown, experts say. The coating acts as insulation, similar to the plastic covering on an electrical cable, and allows for fast bursts of signals around the body and brain. When the sheath deteriorates, signals passing along the neurons in the brain slow down. This means reaction times in the body are slower too."