According to surveys by the Codex Group, a consultant to the publishing industry, 47 percent of 18- to 24-year-old women and 24 percent of same-aged men say most of the books they buy are classified as young adult. The percentage of female Y.A. fans between the ages of 25 and 44 has nearly doubled in the past four years. Today, nearly one in five 35- to 44-year-olds say they most frequently buy Y.A. books. For themselves.
I still do: sometimes favorites from my childhood, and sometimes new things. My reasons are the same as many of the enthusiasts quoted in the article: freshness, engaging characters, and really good plotting. "A lot of contemporary adult literature is characterized by a real distrust of plot. I think young adult fiction is one of the few areas of literature right now where storytelling really thrives." Lev Grossman, book critic for Time.
You can read about the hoopla surrounding the publication of Mockingjay if you're interested in the mainstreaming of YA.