It appears that the world of work, which follows university, has been the main force behind the adult vocabulary gains of the last half-century…Note that in 1953, low-IQ people enhanced their vocabularies over the ages of 17 to 22 far more than low-IQ people did in 2000. I suggest the hypothesis that they were more likely to be settled in apprenticeships or adult jobs in those days than today. Even the high-IQ people increased their vocabularies more between the ages of 17 to 22 in 1953 than in 2000. Apparently being placed in work was more potent than being in a tertiary institution.
Isn’t it also the case that we have been moving to a flatter, simpler English for a long time? Try reading some James Fenimore Cooper. Plus schools are less likely to make you memorize long, classic poems, which is another good way of building vocabulary.