It is possible for relatively smart people to have been socialized into hiding their intelligence (happens to women more than men) and discover later that they are smarter than they thought and perhaps go to university at age 30.
It is possible for people of any intelligence level to suffer brain trauma through motorcycle accident, substance abuse, long covid, lyme, or other disease or injury and become less intelligent, but that is not the norm.
The norm is that if a person is stupid as they approach old age, they most likely have been stupid for as long as they have memories. Everyone gets a bye up to say 3-4 when first conscious memories seem to get laid down, though it certainly is possible to exhibit intelligence before that age.
One consequence is that people like the average maga (not all, but the "normal" probability distribution center group) -- the average maga has been aware their whole lives that they are stupid (to the limited extent that they are self-aware) and thus they carry grudges. They are sick of being stupid and being known to be stupid and so they may bust out with some stupid act like pulling a gun on someone just because they couldn't get their way. If they are white male, they frequently feel entitled to higher positions, more status, and more power than they are capable of or have received.
Note: I have much respect for slow-thinkers who may have difficulty but by patience and time have arrived at basic truths and wisdom. They tend to be hard workers, save money, and raise loving families. "The salt of the earth".