According to the latest research, our brain is fastest at 39 and afterwards declines 'at an accelerating rate.'
The slowdown occurs because of the loss of a fatty sheath which coats the nerve cells, called neurons, during middle age, 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.
Scientists at the University of California, Los Angeles, say that after 40 the body 'loses the battle' to repair the protective sheaths.
The finding was made after researchers tested how quickly men aged from 23 to 80 could tap their index fingers in ten seconds.
The results from the 72 participants showed that both 'had lifespan trajectories that were virtually indistinguishable' - and, significantly, peaked at 39.
