The federal guidelines regarding child safety seats will vary depending on the child's age and size. The sizes and types of car seat your child rides in will grow with them. Rear-facing car seats are recommended for children starting at birth, and they can remain in them until three or four years old unless they outgrow the seat. Depending on what comes first, the child's age or size will determine when they move from a rear-facing car seat to a forward-facing car seat.