A booster seat must be able to accommodate a child until he is big enough to use a seat belt alone. The minimum height and weight accommodations on booster seats vary by manufacturer, but they should all accommodate a child until he is at least 57 inches in height.
The minimum age for a child to use only a seat belt with no booster seat is 8. A child should meet both the age restriction and height restriction before moving to a seat belt. There is no weight restriction on seat belt use, but booster seats do have a maximum weight range.