Consider the effect of age on intellectual ability. Young scientists find that human intellectual abilities decline with age, starting in the late twenties or thirties. However, older scientists doing the same kind of research show that intellectual ability does not decline with age, at least not after you exclude the effects of illness, "cohort" effects, and other technical complexities. Sure, physical and sensory abilities decline ? physical strength, vision, hearing, speed of response ? but certainly not intellectual ability. In fact, older scientists point out, knowledge, or what is called "crystal intelligence," is not only unimpaired, it improves with age.