Depends... You can use a car seat until the child exceeds the listed capacity on the seat you have.
Variable. Car seats are manufactured to safely restrain your child in the event of a car crash. They are designed to hold children of different heights and weights. Check the manufacturers recommended size to determine whether the car seat fits. There is no set age limit, it is a physics problem based on height and weight. So follow the manufacturer's guidelines for safe travel.
Depends on the seat. To the safety tech the term car seat can apply to any safety appliance.There are newer 5point harness seat systems that are designed to fit forward facing (2-3yr+) up to 65-85 lb.These systems offer greater protection for kids in side impact & roll overs. Belt positioning boosters are available to 40-110 lb & satey first offers speciaty seats for larger kids. All below 4'7'' need a safety seat.