U.S. Preventive Services Task Force
The U.S. Preventive Services Task Force issues a recommendation stating that, in consultation with their doctor and pharmacist, and provided the risk of gastrointestinal bleeding and the very rare risk of certain hemorrhagic strokes do not outweigh the benefits in individual cases, that people aged 50 to 70 (especially those aged 50-60 and with a 10 percent risk or higher of cardiovascular disease, or CVD; mindful that the risk of bleeding, which can be dangerous, goes up as one ages) should take low-dose aspirin, for a period of at least 10 years, for preventive benefits against CVD and heart attack, as well as colorectal cancer. The evidence is inconclusive for those not at very high risk who are over 70, and below 40, and there is only weak evidence for prevention of lung cancer, breast cancer, and prostate cancer.
Sources: NY Times Blogs U.S. Preventive Services