When is a good age to use booster car seat?

Generally when your child has outgrown the current carseat. Below will be a detail guideline on when is the best age for booster car seat http://safeseats4kids.aaa.com/faqs/when/when-is-my-child-ready-for-a-booster-seat/

your child's weight and height will determine if a booster car seat is necessary.