Under the age of 8: All children riding in passenger motor vehicles must be in a federally approved child passenger restraint that is properly fastened and secured according to the manufacturer's instructions until they are 8 years old OR 57 inches (ie 4'9") tall.
Under the age of 13: When children outgrow their booster seats (usually around age 8, but more importantly, when they are taller than 57 inches) they must wear a seat belt that is properly adjusted and fastened according to the manufacturer's instructions.
Chapter 90, Section 7AA is subject to primary enforcement by police officers meaning that officers may use a violation of this law as basis for a vehicle stop. No other reason is needed.
Operators and passengers must have their seat belt properly fastened when riding in privately owned vehicles.
Chapter 90, Section 13A is subject to secondary enforcement by police officers meaning that officers may not stop a vehicle based on the fact that an operator or passenger is not wearing a seat belt. A separate reason must exist for the initial stop.