As children grow, how they sit in your car will change. Make sure the car seat you purchase is designed to fit your child’s current size and age. Use these guidelines from safercar.gov:
Birth – 12 Months
Your child under age 1 should always ride in a rear-facing car seat. There are different types of rear-facing car seats. Infant-only seats can only be used rear-facing. Convertible and 3-in-1 car seats typically have higher height and weight limits for the rear-facing position, allowing you to keep your child rear-facing for a longer period of time.
1 – 3 Years
Keep your child rear-facing as long as possible. It’s the best way to keep him or her safe. Your child should remain in a rear-facing car seat until he or she reaches the top height or weight limit allowed by your car seat’s manufacturer. Once your child outgrows the rear-facing car seat, your child is ready to travel in a forward-facing car seat with a harness
4 – 7 Years
Keep your child in a forward-facing car seat with a harness until he or she reaches the top height or weight limit allowed by your car seat’s manufacturer. Once your child outgrows the forward-facing car seat with a harness, it’s time to travel in a booster seat, but still in the back seat.
8 – 12 Years
Keep your child in a booster seat until he or she is big enough to fit in a seat belt properly. For a seat belt to fit properly the lap belt must lie snugly across the upper thighs, not the stomach. The shoulder belt should lie snug across the shoulder and chest and not cross the neck or face. Remember: your child should still ride in the back seat because it’s safer there.