IQ is related to age in two ways. IQ tests were invented to compare a particular child's intellectual performance with that of others of his or her age, for one. IQ is also related to age in that it may decline as people grow older. At least, a central component of IQ appears to decline with age. It may be possible for other components of IQ to actually rise with age, although this is more controversial.
Dr. Alfred Binet designed the first intelligence test. In 1904, he was asked by the French Committee of Public Instruction to help deal with the problem of schoolchildren who seemed slow to learn. To distinguish between lack of ability and other problems that might impede progress in school, Dr. Binet and his associate Dr. Theodore Simon created the first intelligence tests. These tests were designed to measure the "mental age" of a child by comparing his or her score to those of a large group of children. A child might perform on a test as if he were older than his age, or younger. He might, for example, be able to unwrap a piece of candy at age three, when the average age of children who could unwrap a particular type of candy was, say, four. His chronological age would be three; his mental age would be four. The IQ, the intelligence quotient, a concept that came along a little later, was a ratio composed of the child's chronological age and his or her mental age. If a child was ten, but showed the test performance of a fifteen year old, his or her score would be 150, because the IQ was calculated by dividing mental age by chronological age and then multiplying by 100.
Scores derived this way were useless for adults, because after about sixteen the rise in everybody's mental age levels off. So IQ scores are now calculated by testing how far a person's ability to learn, understand, and reason deviates from the norm for his or her age. The farther away from the average performance on the test is, the higher or lower a person's IQ is. Some IQ tests look for deviation by having subjects put together jigsaw puzzles of increasing difficulty, match patterns by assembling blocks, answer vocabulary questions, do math problems, or even explain the meaning of truisms. These test components, of course, measure skills that can be practiced.
So it is reasonable to wonder whether a person whose life includes a lot of "practice" would experience none (or at least less) of the decline in IQ that many studies show comes with age. Could some people even be able to increase their score? To an extent, it seems possible. London taxi-drivers do mental exercise daily with navigation and exploration, and it seems to reshape their brains. Prisoners and other institutionalized persons who live in environments with minimal stimulation do seem to experience mental decline faster than the rest of us. Perhaps sometimes mental activity preserves mental ability.
Exercise is also believed to preserve mental ability, as is social activity. Certain vocabulary scores may be unchanged by aging, or even rise. One interesting theory is that IQ does not really drop with age, rather the ability to demonstrate IQ on a test drops as older people lose their sensory ability to perceive the contrasts necessary to score well.
There are, however, components of some IQ tests that are intended to measure "g", a slippery quality that might be loosely described as "mental quickness". Unfortunately, g does seem to decline with age. Eighty-year-olds are not as quick as fifty-year-olds, who are not as quick as teenagers. For this reason, IQ test scores are adjusted for age. So if your IQ was 115 at age 18, and you weren't tired, nervous, bored, or distracted when you were tested, your IQ will likely be very close to 115 when you are 75.