By the time the lawyers, insurance companies, and big government are done with us, even adults will be required to ride in booster seats.
I'm guessing that the only reason why such a law hasn't already been passed is that it would cause an instant, armed revolt. Children are far easier to push around.
The following is from the Washington State Booster Seat Coalition web page and it clearly states that after 8 years of age you do NOT need a booster.
KEY PROVISIONS of Washington’s Child Restraint law:
. Children under 13 years old be transported in the back seat where it is practical to do so.
. Children up to their 8th birthday, unless they are 4’9” tall (which ever comes first), must ride in a child restraint. (For example a child car seat, booster seat, vest, or other restraint that is federally approved for use in the car.)
. The restraint system must be used correctly according to the car seat AND vehicle manufacturer’s instructions.
. Vehicles equipped with lap-only seat belts are exempt from the requirement to use a booster seat.
. Children 8-years of age or at least 4’9” who wear a seat belt MUST use it correctly (never under the arm or behind the back) or continue to use a booster