When one talks about height and how it relates to success in life or various sports, one almost always means peak height: How tall one was in early adulthood. But when talk about IQ and success in life, do we mean toddler IQ, childhood IQ, adolescent IQ, young adult IQ, middle age IQ, old age IQ or some composite of all of them?

Of course the correlation between IQ at different ages is very high, but part of the reason it is so high is that being a smart child helps you score high as an adult because vocabulary words acquired in childhood might still give you an edge on an adult IQ test. Culture reduced tests (i.e. Gestalt) allow us to measure IQ independently of past learning so perhaps these are best for distinguishing between childhood and adult IQ, but a huge part of intelligence is the ability to learn from the past.

Consider long-term memory. Does it make sense to distinguish between one’s long-term memory in childhood vs old age, since old people remember childhood events? Who has a better long-term memory? The 90-year-old who can remember what she did at age ten but can’t remember what she did at age 89, or vice versa?