As an European, I did never have any IQ test, nor I know anybody who (to my knowledge) was ever administered an IQ test. I looked at some fac-simile IQ tests on the internet, expecially Raven's matrices.
When I began to read online blogs from the United States, I started to see references to the concept of IQ. I am very confused by the fact that the IQ score seems to be treated as a stable, intrinsic charachteristic of an individual (like the height or the visual acuity).
When you costantly practice some task, you usually become better at that task. I imagine that there exists a finite number of ideas required to solve Raven matrices: even when someone invents new Raven matrices for making new IQ tests, he will do so by remixing the ideas used for previous Raven matrices, because -as Cardano said- "there is practically no new idea which one may bring forward".
The IQ score is the result of an exam, much like school grades. But it is generally understood that school grades are influenced by how much effort you put in the preparation for the exam, by how much your family cares for your grades, and so on. I expect school grades to be fairly correlated to income, or to other mesures of "success".
In a hypothetical society in which all children had to learn chess, and being bad at chess was regarded as a shame, I guess that the ELO chess ratings of 17 year olds would be highly correlated with later achievements. Are IQ tests the only exception to the rule that your grade in an exam is influenced by how much you prepare for that exam? Is there a sense in which IQ is a more "intrinsic" quantity than, for example, the AP exam score, or the ELO chess rating?
You should distinguish between "intelligence as a trait of the brain which we are trying to measure" and "the result of using a specific IQ test". Learning the IQ test is like standing on tiptoes when someone measures your height... yes, you will get a higher result, but you didn't actually grow up, you just cheated the test. You can cheat the IQ test and get a higher score e.g. by learning the right answers, but that doesn't increase your intelligence as such.
(And by the way, please do not take online IQ tests, ever. They are practically all scams. Especially those that say they are endorsed by <whatever organization>; they are not.)
Intelligence is not the same as knowledge or skill. As you get older, you naturally get more knowledge, and you get more skills. And the IQ tests adjust for this; they are calibrated for each age group separately, so a 10 years old would get higher IQ than a 20 years old if both answer correctly 15 out of 20 questions.
In practice, IQ is measured by test (because, how else would you measure it?), and you can cheat by specifically preparing for the test. That increases your test score, but not the underlying intelligence. For example, imagine that someone spends years practicing for one IQ test, and then is unexpectedly given a completely different IQ test. This would probably result in their "true IQ" being revealed.
Shortly, you should imagine intelligence as something like "ability to solve new problems". Of course, there is the technical problem that whenever someone designs a test that measures this ability, people can take the test repeatedly or find the questions and answers online, and then the problems are no longer "new" for them. Yes, this is a problem of tests. It does not make the underlying concept of "ability to solve new problems" invalid.