All children under the age of 2 years should be in a full, 5-point restraint car seat facing backwards. Even if your 18-month-old is tall and looks uncomfortable facing backwards, a rear-facing car seat is the absolute safest position for them in the car and they should remain rear-facing until 2 years of age.

At age 2 years, children may begin facing forward in their car seats, but they should still be in a full, 5-point restraint car seat.

Once your child is 4 years of age and 40 pounds, they may transition to a booster seat. Children should remain in a booster seat until they are 4’9” tall (which usually occurs sometime between ages 9-12 years). Once a child is 4’9” tall, he no longer needs a booster seat, but should always use a regular seat belt. Children should not sit in the front seat of a vehicle until they are 13 years of age.

In summary:
Birth-2 years of age: full, 5-point restraint car seat facing backwards
2-4 years of age: full, 5-point restraint car seat facing forwards
Age 4 years (and 40 pounds) until your child is 4’9” tall: booster seat
Age 13 years and above: may sit in the front seat of a vehicle

For more detailed information on car seats and appropriate installation, here is a link to the American Academy of Pediatrics:
https://www.healthychildren.org/English/safety-prevention/on-the-go/pages/Car-Safety-Seats-Information-for-Families.aspx