Well, you can research the topic and you'll see for yourself that after the age of 25 physical and mental performance start to decline. That's the sole reason why people go to school and college before this age, when they are younger, so that they learn as much as they can, while their brain activity is at its peak. With age a person can become wiser, more experienced and calmer which would definitely add up to their intellect, but their mind speed gets more sluggish - science have proved this over and over again. It's slow and subtle process, one may not notice it instantly. It also depends on one's genes, stress levels, levels of exercise and nutrition how fast would this decline happen. Why do you think people get dementia and Alzheimer's when they become old?