Less than 4 feet 9 inches tall.
1 Generally, kids need to use a booster until they are at least 4 feet 9 inches tall and weigh between 80 and 100 pounds. 2  For most kids, they will be between ages 8 to 12 years old. 3  Talk with everyone who drives your kids so they understand that booster seat use is a must when your child is in their vehicles.  If all back seats are occupied by children under 4, then a child under 4 may ride in a car seat in the front seat. A child in a rear-facing car seat may only ride in the front seat if the airbag is turned off.Children to be properly buckled in a car seat or booster seat until they are 8 years old or 4’9” tall. Children must ride in a seat until they reach the age requirement or the height requirement, whichever comes first.  1 Generally, kids need to use a booster seat until they are at least 4 feet 9 inches tall and weigh between 80 and 100 pounds. 2  For most kids, they will be between ages 8 to 12 year old. 3  We know that when adults wear seat belts, kids wear seat belts.  Boost Your Safety. Booster seats are the most overlooked part of car seat safety. The goal of a booster seat is to lessen the risk of injury by helping the seat belt fit properly. Booster seats are recommended for children 4-7 years old who are less than 4 feet 9 inches tall. When using a booster seat, remember the following: Children no longer need to use a booster seat once seat belts fit them properly. Seat belts fit properly when the lap belt lays across the upper thighs (not the stomach) and the shoulder belt lays across the chest (not the neck). The recommended height for proper seat belt fit is 57 inches tall. Buckling children in age-and size-appropriate car seats, booster seats, and seat belts reduces the risk of serious and fatal injuries: 1  2. 2  Booster seat use reduces the risk for serious injury by 45% for children aged 4–8 years when compared with seat belt use alone. 3  3. Phase 3. Booster Seats. After age 4 and 40+ pounds, children can ride in a booster seat with the adult lap and shoulder belt until the adult safety belt will fit them properly (usually when the child is 4'9 tall, 10 12 years old). MUST have a lap/shoulder belt to use a booster seat.  Does my 8-year-old still need to ride in a booster seat?... Can my son ride in a forward-facing car seat if he is over 1 year old?... What do I do if my daughter has outgrown the height restriction for her car seat, but not the weight restriction?... Should my 60 lb. child ride in the front seat or the back seat?...  Boosters Are For Big Kids Most kids need to ride in a booster seat from about age 4 until age 10-12. If your child isn’t using a booster, try the simple test below the next time you ride together in the car. You may find that your child is not yet ready to use a safety belt without a booster.  Laws aside, kids aren't ready to make the switch from a booster seat to seat belts until they're at least 4 feet 9 inches tall and can pass the five-step test below — typically between ages 8 and 10. 
http://www.safekids.org/safetytips/field_age/big-kids-5%25E2%2580%25939-years/field_risks/booster-seat http://www.michigan.gov/documents/msp/booster_flyers_361887_7.pdf http://www.safekids.org/safetytips/field_age/big-kids-5%E2%80%939-years/field_risks/booster-seat http://ocp.ga.gov/consumer-topics/child-car-seats http://www.cdc.gov/MotorVehicleSafety/Child_Passenger_Safety/CPS-Factsheet.html https://www.dps.texas.gov/director_staff/public_information/carseat.htm https://www.edgarsnyder.com/car-accident/defective-products/car-seat/car-seat-laws/ http://carseat.org/Boosters/630.pdf http://www.babycenter.com/404_when-can-my-child-switch-from-a-booster-seat-to-seat-belts-a_71328.bc
how tall does a child need to be for a booster seat