Bertie the Bunyip writes:

IQ isn't really age related. though it is true that capacity increases with
age, the measurement is usually like/like otherwise it's kind of pointless.

Capacity does not increase or decrease significantly with age. Someone who is
smart in childhood will be smart in old age as well. Someone who is stupid as
an adult was also stupid as a child (excluding pathology).

Extremely poor living conditions very early in life can prevent a person from
coming close to his genetically-determined IQ limit. Likewise, some types of
illness (especially CVAs) can diminish IQ scores temporarily or permanently.
But healthy people in normal environments tend to reach IQs close to their
genetic programming and these tend to remain fairly constant over their
lifetimes.