Mammograms are more beneficial for women 50 to 74 years old than for younger women, while the benefits for women 75 and older are unclear, a federal panel that evaluates medical practices said Monday.

The U.S. Preventive Services Task Force examined clinical studies on breast-cancer screening, concluding that “some women in their 40s will benefit from mammography” but that “most will not, while others will be harmed.” The panel said those hurt include women who undergo surgery, radiation or chemotherapy for cancers that never...