After the child has reached the upper weight limit of the car seat and is AT LEAST 4 years old. A lot of seats harness until 50 pounds or more. My son's goes to 50 pounds. If he's anything like his older brother, he won't reach that until 5 or 6 years old. It is safest to keep your child harnesses as long as possible.
Keep them harnessed!!! My kids will stay harnessed until atleast 6 years old. Our seats harness to 65lbs. My 3.5 yr old is 30lbs even and my 1.5 year old is 21 lbs, they will stay harnessed for there safety until 65lbs.