try Carseat Regulations, thats what they are here but it's all based on seat weights and stuff I found a canadian site, but canadian sites probably just show up first for me cause i'm in Canada
here is what it says--
Birth to 9 kg (20 lb.)
"Itís best to keep your child rear-facing until they are at least one year old or until they have reached either the maximum height or weight limits of the rear-facing seat."
9 to 18 kg (20-40 lb.)
Preschool - 8 years old :
Between 18 - 36 kg (40ó80 lb.)
"A child can start using a seatbelt alone once any one
of the following criteria is met:
- child turns eight years old
- child weighs 36 kg (80 lb.)
- child is 145 cm (4 feet-9 inches) tall. "
basicly with all the "regulations" it depends on the child, weight not just there age