"Is there an age limit for child-safety seats?"
If by age limit you meant the seat itself, Cars.com editor Jennifer Geiger advises that child-safety seats have a shelf life of six years. Car seats are required to show either an expiration date or a date of manufacture, and they should be replaced after six years because materials in the seats can weaken over time and from use.
If you were asking about the ages of children, Geiger, a certified child-safety seat installation technician, says the laws are different in each state, but as a general rule it isn't safe for a child to be secured only with a seat belt until they are at least 8 years old or 4 feet 9 inches tall.
The Insurance Institute for Highway Safety maintains a list of child-restraint laws by state that should explain the requirements where you live.
As kids grow, parents have to decide when it's time to transition from an infant seat to a convertible seat, then to a booster and — finally — when they can be secured with just a seat belt. Here are helpful tips on making that first step out of an infant seat and then taking the final big step out of a booster.Related
Car Seat Basics Part Two: From Infant to Convertible Seat
Car Seat Basics Part Three: Beyond the Booster
More on Child Safety from IIHS