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Multiple Levels of Adult Cognitive Development Along with the increase in overall complexity of adults' cognitive development, both developmental research and everyday observations indicate that adults show multiple levels of cognitive development, not performance at one .
Intellectual Development: Age 17–45 This question has plagued psychologists for decades. Cross‐sectional studies of IQ tend to show that young adults perform better than middle‐aged or older adults, while longitudinal studies of IQ appear to indicate that people increase in intelligence through the decades, at least until their 50s.