My daughter is 3 and she is still in her first carseat. Its a large safety first convertible carseat. Converts from rear facing to forward facing. (i only say that because i remember being like what is a convertible carseat?? lol)
My sons car seat holds from 5-80lbs its convertable car seat I got for 95$ att sams club. I love it and won't have to buy a new car seat for him ever again. it'll follow him until he doesn't need a seat anymore. Which is roughly around 7 it depends on your childs weight and height though.
The car seats go by weight... some are adjustable also.. but the infant car seat my daughter used hers till she was 1 years old..
I bought a travel system that has a carseat that.goes up to 30 lbs, once my baby girl gets to 30lbs I have to buy a bigger carseat. Infants use smaller carseats than older babies. Then once they hit a certain weight they go to booster seats but that's much farther down the road :)