[Arun2009]

There are three strong factors that come to mind:

(1) Physical fitness. These same years have seen the explosive growth of obesity and related lifestyle diseases. A greater percentage of people in first world countries (and increasingly elsewhere) are either obese or overweight. I recall reading that one way to keep your mind sharp is to be physically fit (cf: "Spark: The Revolutionary New Science of Exercise and the Brain" by Ratey and Hagerman). The corollary could be that if you are not physically fit, your IQ will suffer.

(2) Pollution. Air pollution has been shown to affect IQ scores (https://www.unep.org/news-and-stories/story/air-pollution-li...). It could be that greater pollution has been causing lower IQ scores.

(3) Sleep. I recall reading that people get lesser high quality sleep than they used to. It has very clearly become easier to stay up late today. Poor sleep is really bad for you for a number of reasons, including your cognitive performance and brain health.