On Friday, Governor Patrick signed St.2008, c.79, An Act Relative to Child Passenger Safety. Under the new law, effective May 1, 2008, "No child under the age of eight and measuring less than fifty-seven inches in height shall ride as a passenger in a motor vehicle on any way unless such child is properly fastened and secured, according to the manufacturer’s instructions, by a child passenger restraint." The standards for child safety seats are set out in 49 CFR 571.213.
We've had questions about whether the law applies to children over eight years old but less than 4' 9". You can see from the excerpt above that the requirement is for children who are both under eight AND less than 4'9".
You can find more information on issues related to driving at Mass. Law About Traffic Violations.