To give a few excerpts, Simmons Market Research states
... an estimated 18.1 million Americans participated in karate or some other form of martial art at least once in the past year. Included in that estimate are 9.4 million adults, 5.5 million teenagers and 3.2 million kids.
They then break into further statistics for age groups, gender, and affluence. For example:
An estimated 5 percent of adults say they participated in martial arts last year at least once, and a quarter of those (28 percent) say they do martial arts every chance they get. Surprisingly, this bunch is fairly evenly split between men (52 percent) and women (48 percent). But for the most part, participants are young. Sixty-three percent are between 18 and 34, compared with 25 percent who are between 35 and 49 and 11 percent who are 50 or older.
Interestingly, Asian American adults are no more likely to participate in martial arts than are whites; around 5 percent of each group is involved in the sport. Blacks, on the other hand, are more likely than whites or Asians to engage in martial arts, with 7 percent saying they have participated in the sport at least once in the past year.
And, indeed, income is a definite factor:
Alas, karate lessons don't come cheap, and the likelihood that a child will partake in martial arts increases proportionately with the income of his or her parents. Fifteen percent of kids whose parents earn $75,000 or more a year participate in martial arts, compared with 13 percent of those whose parents make between $50,000 and $75,000. Only 10 percent of kids whose parents earn $50,000 or less participate in the sport.
Sports & Fitness Industry Association apparently has a more comprehensive and current report, but a subscription is required to access it.