A baby usually stays in an infant car seat until they are 12 months to 18 months of age depending on the weight and height of the baby.
The baby needs to remain in the infant car seat until they get too big for the infant car seat.
Then after they grow out of the infant car seat the baby will need to move up to a Forward Facing toddler car seat which has the harness as well.
Then they will need to remain in that forward facing toddler car seat until they get too big for that car seat which can be when they reach 4 years old or above.
The laws also vary from state to state but after the toddler grows out of the toddler car seat then they need to move up to a booster car seat that raises the height of the child so the seat belt fits properly.
The child may then need to ride in a booster seat until 6 years old or 7 years old depending on the child's height and the laws in your states.
Some states require the child to be in a backless booster seat until 8 years old but some don't require that.