Answer:
no its not fixed.
Step-by-step explanation:
it depends on our definition of intelligence, I believe there is many many different kinds of intelligences, so its better to say "intelligenceS" not only IQ, and IQ is a very reduced and limited definition of intelligence, and focusing on it is dangerous. because is against the idea of equality (in law and opportunities, of course all people are different and not equal in all skills abilities).
BTW in my opinion intelligences can change, based on many things: our diet, fatigue, bein bored or not, being sad or happy. and also learning makes us more intelligent. that's why we try to learn more and more and after that we can solve problems that we could not do it before.
The danger of focus on IQ, is giving a privilege to someone as a genius, and leave other less talented people, and it helps to make narcissism. fame, wealth and power to someone, who didn't choose or made that talent and it seems its a untouchable level, kind of gift from god. and this POV fuels inequality.
(based on determinism, we do not choose anything, but changing abilities are possible, although we can not change the chain of causes before ourselves, but it is possible to participate in future causes).