From the Crohn's and Colitis Foundation of America:
It is estimated that as many as one million Americans have IBD-with that number evenly split between Crohn's disease and ulcerative colitis. Males and females appear to be affected equally.
Ulcerative colitis is predominantly a disease of the young. Most cases are diagnosed before age 30, although the disease can occur at any age -- including the later decades of life. Indeed, a much smaller number of patients may develop the disease between the ages of 50 and 70. There is a greater incidence of ulcerative colitis among whites than in non-whites, and a higher incidence in Jews than in non-Jews.
They also estimate the number of children and teenagers in the US with IBD to be 100,000.