There’s a lot of research out there about the general flexibility of the brain to adapt to whatever stimulus you pump into it. For example taxi cab drivers have larger areas in their hippocampus dedicated to place cells relative to the general population [1]. There’s also all kinds of work studying general flexibility of the brain in response to novel stimulus like the visual cortex of blind people being dedicated to auditory processing [2 is a broad review]. I guess you could argue that the ability to be flexible is intelligence but the timescales over which a brain functionally changes is longer than a general day to day flexibility. Maybe some brains come into an initial state that’s more predisposed to the set of properties that we deem as “intelligence” but development is so stimulus dependent that I think this definition of a fixed intelligence is functionally meaningless. There are definitely differences in what you can learn as you age but anyone stating we have any causal measure of innate intelligence is claiming far more than we actually have evidence for. We have far more evidence to suggest that we can train at least the appearance and usage of “intelligence”. After all no one is born capable of formal logical reasoning and it must be taught [3,4 kind of weak citations foe this claim but there’s a lot to suggest this that I don’t feel like digging up]

[1] https://pubmed.ncbi.nlm.nih.gov/17024677/ [2] https://www.annualreviews.org/content/journals/10.1146/annur... [3] https://psychologyfor.com/wason-selection-task-what-it-is-an... [4] https://www.tandfonline.com/doi/full/10.1080/14794802.2021.1...