I understand a person’s intelligence is proportionately stable to age forgoing injuries and abuse. I’m not debating this.
IQ tests are not necessarily pure evaluations of intelligence. An IQ test based on foreign material and reason will yield lower scores than one that is not. The difference here is knowledge and experience, not intelligence.
You are speaking to the external validity of the test, yes that can vary quite a bit. I’m sure that if China were giving the tests in Cantonese, Americans probably would score a little lower than their workers.