How does aging affect intelligence?

a) Brains in older people tend not to work as fast. But older people are often more knowledgeable than younger people.
b) Older people are more intelligent because they have been using their brains for so many years.
c) Older people are significantly less knowledgeable than they were when they were younger.

Evidence suggests that knowledge levels remain stable or increase well into late adulthood. In one study, older subjects knew much more than younger adults did about an array of subjects. However, the speed at which the brain processes information does seem to decline with age. Healthy, older adults took longer to respond to timed questions than their younger intellectual peers.