James Flynn explains how retirement might impact IQ:
The wisdom always was that the brighter you were, the less your mental abilities declined in old age. I found that was an oversimplification. It is true of verbal intelligence. The brighter you are, the more you get a bonus for verbal skills. I call that a “bright bonus.” Your vocabulary declines at a much less steep rate in old age than an ordinary or below average person. But to my amazement I found that for analytic abilities it was just the reverse. There is a “bright tax.” The brighter you are, the quicker after the age of 65 you have a downward curve for your analytic abilities. For a bright person, you go downhill faster than an average person.
This raises an interesting question. Is it something to do with the aging brain, or does it have to do with environment? It could be that a good analytic brain is like a high performance sports car; it just requires more maintenance, and in old age, the body can’t give it. That would be a physiological explanation; the bright brain requires sustenance from the body, which as the body ages is no longer forthcoming. The environmental explanation would be that we use our analytic abilities mainly at work. That means that if a bright person is in a cognitively demanding profession, they are like an athlete; they build up a big exercise advantage over the average person, who has a humdrum job. Then, retirement would be a leveler. That is, if you give up work at 65, you are like an athlete who is retired from competition. You no longer have that exercise advantage of your analytic abilities that work affords. We don’t really know which of these things is true. It could be that they are both true to some degree.