Wearing a bike helmet could save your life. According to data from the U.S. Department of Transportation's Fatality Analysis Reporting System, in 2009, 91% of people killed while riding a bicycle were not wearing helmets.
Massachusetts State Law requires that if you are 16 years old or younger, you must wear a helmet that meets ANSI requirements while riding a bike on a public way, even as a passenger. The helmet must fit your head and the chin strap must be fastened. Violations of the law can result in impounding of the bicycle for up to 15 days.
More information regarding Massachusetts bicycle laws can be found in Massachusetts General Law, Chapter 85, Section 11b.