I think the whole point of IQ test is being stable independent of age.

It seems like a goal of IQ is to assign a single, unchanging score to someone's cognitive ability, but it is a severely flawed concept. The test should not be called "intelligence quotient" but something like "cognitive battery" instead.

You don't have an IQ. It's a test that you take that can provide some useful information, but it isn't a fixed score, and "intelligence" (a word that is too ambiguous to use without careful definition) can't be reduced to a single number.

There's a difference as it were between, roughly speaking, stored knowledge and being able to figure things out on the fly, but multi-factor analysis says there's only one factor for being good at IQ tests, oddly enough. And every test of doing mental tasks we know how to create correlates strongly with any other test. There's a good explanation about how the tests work here:

https://www.youtube.com/watch?v=Nfg0hfrQMFM

I'm not saying that the tests are useless, just that they should be renamed, and culture should look at them a little differently.

I would agree with you if you're saying that they're not at all a measure of human worth. I don't get the 'rename' comment, though.

If we don't call one's skill at mental tasks 'intelligence', just how should we label it?

"Intelligence" is poorly defined in most conversations. The nature of the word and concepts around it lead to problems, even for people who can score high on IQ tests.

It will take me a long time to write out my thoughts completely, but I plan to do it at some point.

Mostly it seems to be quick and adaptable thinking, if I understand the test results right. The colloquial usage is all over the place, as usual, but the tests strongly correlate with things like being able to attain advanced education and ability to be successful. Some of it appears to be inherent, whether it's clock speed/RAM equivalent for our brains or what, because it survives twins raised in completely different homes (including large economic differences). That said, there are certainly environmental effects, too, especially in the negative direction (e.g. lead exposure).

Originally though IQ was calculated by dividing by age, now it's calculated by comparing with those in the same age group. So any random child outscoring a famous scientist doesn't seem to be that significant.

I think I tested around 30 (apparently still insecure enough back then to care, these days I care what my dogs think more than I care what people think :)