The annual General Social Survey of a representative sample of Americans includes a 10 question vocabulary test, which correlates with IQ tests at the 0.7 level. Inductivist finds that recent college graduates had lousier vocabularies than previous college graduates, as scored on an IQ like scale where 100 is the average and 15 is the standard deviation.
Mean IQ--white college grads
In case you're wondering, this isn't an effect of people getting better vocabularies as they get older. Inductivist explains:
Except for this decade, all of the other estimates are based on data from Americans who were college age the decade before, so any mean increase due to age should be the same across decades.