Rates of colorectal cancer, which overall have been declining for decades in the United States, are instead rising sharply among young and middle-aged adults, according to a new study that startled researchers and is sparking questions about whether screening should start earlier than age 50. The study, published Tuesday in the Journal of the National Cancer Institute, found that between the mid-1980s and 2013, colon cancer rates increased about 1 to 2 percent per year for people in their 20s and 30s. Rates for middle-aged adults also rose, but at a slower pace.