IQs are not fixed for life, although generally they stay within a range, neither jumping or dropping more than a certain number of points (10 - 20?). There are cases of IQs fluctuating more than that, but it's pretty rare.
Not really. A 10 year old with a mental age of 15 and an IQ of 150 is a great deal smarter than a 20 year old with a mental age of 20 (higher than the child's) and an IQ of 100. The 10 year old has had half the life experience and therefore does not know or understand many things that are simple for the 20 year old, but give both of them something new for them both and pretty difficult and the 10 year old will learn it a great deal faster and understand it more completely than the 20 year old. The IQ isn't adjusted for age, the test is designed for the age. IQ tests are also only valid within a particular culture. We also need to remember that there are different types of intelligence.
Coldwater_Canuck:In that case, does saying a kid has an IQ higher than an adult have any meaning other than "eventually this kid will be smarter than this adult"?
Not really. A 10 year old with a mental age of 15 and an IQ of 150 is a great deal smarter than a 20 year old with a mental age of 20 (higher than the child's) and an IQ of 100. The 10 year old has had half the life experience and therefore does not know or understand many things that are simple for the 20 year old, but give both of them something new for them both and pretty difficult and the 10 year old will learn it a great deal faster and understand it more completely than the 20 year old. The IQ isn't adjusted for age, the test is designed for the age. IQ tests are also only valid within a particular culture. We also need to remember that there are different types of intelligence.