If you're confused, I would recommend going to a proper baby shop rather than Kmart or the like, as the staff there should know what the regulations are and be able to help you choose the correct car seat. They also normally have someone that is trained to fit the car seat correctly. DD is 3.4 years old, and is quite tall for her age, and she is in a Safe n Sound Maxi rider AHR - the head rest is adjustable, and it apparently should last until she is 7 years old.