A new national report reveals that 45.9 million American adults aged 18 or older, or 20 percent of this age group, experienced mental illness in the past year. The rate of mental illness was more than twice as high among those aged 18 to 25 (29.9 percent) than among those aged 50 and older (14.3 percent). Adult women were also more likely than men to have experienced mental illness in the past year (23 percent versus 16.8 percent).