Considerable research has found that fluid intelligence declines with age. There is a 2006 paper "Age differences in fluid intelligence: Contributions of general slowing and frontal decline".
We know that 50-year-olds are not as strong physically as 20-year-olds. The brain is an organ, and it would be surprising if every part of the body except the brain worked less well as one aged.
We know that 50-year-olds are not as strong physically as 20-year-olds. The brain is an organ, and it would be surprising if every part of the body except the brain worked less well as one aged.