The LATCH system has a maximum combined weight of the carseat plus child of 65 pounds. As a result, the threshold varies with the weight of your carseat and weight of your child, but a typical 4- or 5-year old probably should be using the belt.
Personally, I don't think that was a good threshold to set. I agree that the LATCH anchors are better. In 2003, I suppose people were mostly using carseats for infants and toddlers in little 10-pound styrofoam shells, now we know kids are safer in carseats until they're significantly older and heavier, and they're in 25-pound adjustable, heavily bolstered bucket seat style carseats.
https://csftl.org/whats-the-deal-with-weight-limits/
Personally, I don't think that was a good threshold to set. I agree that the LATCH anchors are better. In 2003, I suppose people were mostly using carseats for infants and toddlers in little 10-pound styrofoam shells, now we know kids are safer in carseats until they're significantly older and heavier, and they're in 25-pound adjustable, heavily bolstered bucket seat style carseats.