Totally agree. Rohwedder and Willis posit two hypotheses:
"Two Hypotheses about Mental Retirement
Why does retirement cause cognitive decline? One hypothesis is that workers engage in more mental exercise than retirees because work environments provide more cognitively challenging and stimulating environments than does the non-work environment. Indeed, recommendations that retirees maintain an engaged lifestyle, pursuing leisure activities such as playing bridge or doing crossword puzzles—“using it” to prevent “losing it”—implicitly assume that the life of a retiree may lack cognitive stimulation unless deliberate offsetting actions are taken. Thus, we shall call this explanation of the mental retirement effect the “unengaged lifestyle hypothesis.”
As we discussed at the beginning of the paper, Salthouse (2006) offers reasons to be skeptical of evidence purporting to demonstrate that mental exercise reduces the rate of cognitive decline. Nonetheless, we believe that the unengaged lifestyle hypothesis may be a plausible explanation for the mental retirement effect for several reasons. First, unlike many of the interventions discussed by Salthouse, like crossword puzzles and card games, retirement represents a major change in a person’s lifestyle and activities and thus affords the potential for a large effect. Second, the range of cross-country variation in age of retirement due to differences in policy is also large. Finally, the ten-year span between ages 50-54 and 60-64 in Figure 1 is long enough to indicate that the mental retirement effect represents a change in the rate of cognitive decline, rather than a short-term effect of retirement itself.
Human capital theory suggests another mechanism that might produce a mental retirement effect: the prospect of early retirement may bring about a decreased level of mental exercise while still on the job. Since the human capital production function requires a person to combine cognitive ability, stock of knowledge and effort to produce additional human capital, mental exercise tends to be an increasing function of the volume of investment. For workers late in their careers, the value of continuing to build work-related human capital is very sensitive to the length of the remaining working life. For example, a 50-year-old worker in the United States who expects to work until 65 has a much greater incentive to continue investing in human capital than does a worker in Italy who expects to retire at 57. Thus, we hypothesize that differences in retirement incentives across countries create a reduction in mental exercise at work that may begin well before actual retirement. We call this the “on-the-job” retirement effect.
Taken together, our two hypotheses suggest that variations in mental exercise associated with both the work environment before retirement and the home environment after retirement may causally influence components of fluid intelligence such as the memory recall measures contained in the HRS, ELSA and SHARE data used in this paper. These hypotheses would lack plausibility under the hereditarian view expressed most famously by Hernnstein and Murray (1994) that intelligence is largely fixed by genetic inheritance and is immutable. There is, however, a large body of evidence in both economics and psychology showing that the education-ability relationship is bi-directional, with education having a causal impact on ability as well as the converse. (See, for example, Neal and Johnson, 1996; Ceci, 1991).
More broadly, beginning with Flynn (1984, 1987), the existence of substantial gains in cognitive test performance across cohorts have been found in many countries, with mean-level gains on the order of one standard deviation in IQ per generation.7 Dickens and Flynn (2001) have developed a theoretical model to explain “Flynn Effects.” In their model, an individual with a small genetic advantage—say, being slightly taller—has a slightly higher chance of entering an environment that would amplify talents associated with that advantage—being picked to play basketball–so that, in effect, genes select their own environment. Feedback loops over the life cycle strengthen this relationship. In an extension of the Dickens-Flynn model to multiple abilities, Dickens (2007) states, “A general intelligence factor arises in the model because people who are better at any cognitive skill are more likely to end up in environments that cause them to develop all skills.” 8
While such selection and feedback effects are clearly critical in early childhood and during formal education, occupational choice and investment in on-the-job training mean that such effects may continue throughout the working life cycle and into retirement. In the next section, we investigate whether differences in cognitive stimulation of the environment created by variations in retirement policies generate differences in cognitive abilities across countries."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2958696/