It’s time to switch to a convertible seat when the top of your baby's head is within 1 inch of the top of the car seat shell.
If you are considering a used seat, only accept it if you the previous owner to give you an honest crash history and you can verify the seat hasn't been part of a recall. when to buy a new car seat
Car seats aren't "buy once, use forever." They typically expire after the date of manufacture. It’s time to switch to a convertible seat