How old does a child have to be to be able to sit in a booster seat in the state of Virginia?

"Booster seats are necessary when a child outgrows his/her child safety seat, usually after 40 pounds or age 4. Children do not fit in adult shoulder/lap belts (without a booster seat) until they are 58 inches tall (with a sitting height of 29 inches) and weigh 80 pounds, according to the Centers for Disease Control and Prevention (CDC)." Here's a helpful link... http://www.healthsystem.virginia.edu/UVAHealth/peds_safety/mvseats.cfm