The laws regarding which children are required to sit in booster seats while riding in an automobile vary greatly from state to state. Kids of the same age also vary enormously in size. This has Consumer Reports asking whether a single federal law booster seat law based on the height of the child would work better.
As discussed by Consumer Reports, all states require safety seats for children under one year old. Every state except Arizona, Florida and South Dakota has enacted booster seat requirements for some children.
A quick look at the child safety seat laws of all states shows the extreme differences in booster seat laws. Some base it solely on age, others on age and weight, and yet others on age and height.
As advised by Consumer Reports' Safety blog, parents wondering if going without a booster seat is okay should ask the following questions:
No to any of those means yes to the booster seat.
Unfortunately, the wide variance of state laws means you still need to check your state's requirements as well.
From Consumer Reports' point of view, since height is most often the determining factor in how a seat belt fits on a child, the best solution would be a single federal booster seat requirement for all kids under 4'9" (57 inches).