Keep babies rear facing as long as possible. Keep small children in a seat with a harness as long as possible. Keep older children in a belt-positioning booster as long as possible. It may be more economical to use a convertible seat from birth, but the slots for the shoulder and crotch straps generally are not as close to the infant's body. Babies must ride rear facing until they are at least one year old to protect the spine, possibly preventing death or lifelong disability.
Babies have heavy heads and weak necks with soft bones and stretchy ligaments.
At about one year, the bones start to harden, and the baby gradually becomes less vulnerable. Older babies usually outgrow infant-only seats before they are old enough to face forward at one year. Otherwise, the baby's head may not be fully contained within the seat for best protection in a crash. When selecting a convertible seat, be sure to check the maximum rear-facing weight, which varies from 20 to 35 pounds. The average baby weighs 20 pounds at nine months and 22 pounds at 12 months.