Depends on child. The booster is designed so the waist belt does not ride up on the abdomen in a head on crash.These seats offer almost no protection in a side impact or roll over. We now have 5 point harness systems that protect better and fit kids up to 65-85 lb. I keep kids in a harness system as long as possible and use boosters for the 6-12yo.
Usually 4 years old. Due to the maturity of the skeletal system, switching from 5 point restraints to a booster is done at 4 years old, and 40 pounds. For many states, that is the law, and currently the american academy of pediatrics recommendation. They should stay in the booster until 8 years old, again by aap recommendations, or tall enough for the seatbelt to fit correctly across the chest.