According to developmental psychologist k. warner schaie, inductive reasoning, spatial orientation, perceptual speed, and verbal memory begin to gradually decline beginning around age 25 30 40 50

I believe the answer is: 25

According to Schaeia, the decline could not be reversed but the process can be slowed down/decelerate. The things that most people can do to decelerate the process are by eating proper food that provide enough nutrition for the brain and to constantly use and exercise the brain.