Quick answer
The switch usually happens after the forward-facing harness is outgrown, not when a child reaches a birthday.
The switch point
A child usually moves from a forward-facing car seat to a booster after outgrowing the forward-facing harness height or weight limit. NHTSA describes this stage clearly: once a child outgrows the forward-facing seat with a harness, the next step is a booster in the back seat until the seat belt fits properly.
Why not switch early
A forward-facing harness holds the child in position. A booster depends on the child staying in position with the vehicle belt. That is a big behavior change. A child who leans forward, twists, sleeps sideways, or moves the shoulder belt behind the back may not be ready for a booster even if they meet a minimum age or weight.
What to check before switching
Check the harness limit in the manual, the child's shoulder position relative to the harness slots, the top of the ears relative to the seat shell where applicable, and the booster minimums. Also check your state law because some states define child restraint requirements differently.
After the switch
After moving to a booster, keep the child in the back seat and keep testing belt fit. The booster should position the lap belt low and the shoulder belt across the chest. If it does not, try a different seating position or booster style before assuming the child is ready for a seat belt alone.
Before you decide
Do not make this decision from one number on a search result. Check the child restraint label, the car seat manual, the vehicle manual, and your state law. Then test the setup with the child actually sitting in the vehicle. A booster or seat belt that looks fine in theory can fit poorly in a specific back seat because belt anchors, seat depth, buckle stalks, and head restraints vary by vehicle.
If this is a legal question, start with your state page. If this is a safety question, start with the core guide linked below. The safest answer is usually the one that satisfies the law, matches the product instructions, and keeps the belt or harness positioned correctly for the full ride.
When to get hands-on help
Get help from a certified Child Passenger Safety Technician if the belt crosses the neck or face, the lap belt sits on the stomach, the booster slides around, the child cannot stay positioned, the vehicle only has a lap belt in the seating position you want to use, or you are unsure whether a forward-facing seat has been outgrown. A short in-person check can catch problems that are hard to see from a product photo or a general article.
Helpful next steps
Sources used for this topic
This article follows the same source hierarchy as the core guides: NHTSA, CDC, AAP/HealthyChildren, IIHS, federal standards, and state law pages. Check the linked core guides for the source list and state-specific notes.