Intelligence is misunderstood by a majority of people.
People think intelligence is this static thing that you are born with innately, and they cling to this belief, often to explain those that are so much better as natural talent/intelligence. This is mistaken.
Intelligence is a combination of speed of association, mental framework, and experience/exposure to differing and disagreeing ideas.
Believe it or not, you are more intelligent today, than you were when you were at 15 years old. The things you picked up along the way towards doing things successfully made you more intelligent, and there are things that can make you less intelligent too. Trauma and psychological stress as an example.
There are thresholds where people can't become smarter because the speed of association is so slow or the interference so high, that it doesn't provide a connection of ideas. IQ of less than 83 is the cutoff the military uses.
The framework/mental model you use is quite important as well, because intelligent people aren't delusional. They are methodical, and test the structures they use, and are right more often than they are wrong by a significant margin.
People who have been tortured, had significant trauma, or people who were forced to learn by rote false things, and then forced to unlearn those things repeatedly, all have significantly slower speed of association, which is a precursor for comprehension.
There are physiological factors that impact these things as well. How healthy you are, whether you take supplements, exercise regularly...
People are protean by nature, able to overcome quite a lot of deficiencies; but often not everything, there are limits; and things that can damage you permanently in life, sometimes without you even realizing it. Those things are the most insidious.