Actually IQ does tend to be “stable” over a lifetime (I remember that much from intro to psychological testing class from college). People with high IQ tests as children TEND to also test high in adulthood (baring any sort of physical abnormalities or developmental problems, for instance a genius kid might have an abusive home life that effects them negatively, or some physical accident could cause brain damage or something).

One of the problems with testing kids in school for IQ though is a lot of it can come down to the person administering the test. A friend and I were both tested for the gifted program in grade school by the same county-wide test administrator. Both of us “failed” by a few points. Years later we were talking about it and we’re both like “yeah were you tested by ms. $name? She had a shitty way of asking questions”. Like she would ask “what does this word mean” and then mispronounce the word so it sounded like something else. So you’d give the definition for something else completely and she’d look at you like you were stupid, when it was her that didn’t know what word she was asking about lol. My friend, who didn’t get into the gifted program either, is legit one of the smartest people I’ve ever met, got a 32 on his ACT the first time he took it, got a masters in CSC without even trying, and works as a software engineer for one of the biggest streaming platforms in the world now.

IQ alone though isn’t a great indicator for success over a lifetime. It’s more an indicator for potential all other things being even. A better indicator is “emotional/ interpersonal IQ”. Individuals who are good at being “people persons” tend to do better even if they’re not quite as smart (because as any developer who was ever had to deal with clients directly knows, the best possible decision isn’t always the one that the client ends up making).