Your definition of literacy would include first graders. But let's think about teens.
Some studies indicate that the vocabulary of the typical American teen
of today is less than half the size of the vocabulary of a teenager in the 1950s. It is speculated that the influences of television, computer games, and other entertainment media are largely responsible.
Fewer people read for pleasure these days, and reading books has always been a great vocabulary-booster. Widespread "pop" entertainment has brought a unity of cultural reference to our society, along with a plenitude of simple catchphrases which make it unnecessary to create original sentences. Rather than saying "I am relatively indifferent to this situation," it is easier (and less taxing on the mind) to say "Whatever."
Although slang has always been part of human speech, slang which serves as expressive shorthand has become more prevalent, perhaps because the omnipresence of television has homogenized us, creating a near-universal understanding of slang phrases which might have remained regional in times past.
"Utne Reader (July-August 2000, 'Like Whatever,' pages 28-9.) tells us
that the typical American teenager of the 50s had a vocabulary of
25,000 words; the teenager of today 10,000."
By comparison, it is estimated that typical college graduates have an active vocabulary of 60,000 active words (which they use) and 75,000 passive (which they understand).