Due to evidence showing that a child's vertebrae is connected via cartilage not ossified bone, this does not happen until roughly age 4-6. A severe crash could result in the cartilage stretching and rupturing the spinal cord. It is 5 times safer for an under 4 to travel rearwards.
Due to the amount of severe injuries or deaths caused by babies and toddlers forward facing from far too young an age. If the legal requirement is raised it could save money for the NHS and most importantly the loss of life or life changing injuries to innocent babies and toddlers. There is only an 8% risk of severe injury in a car crash if a child rearward faces compared to 40% of severe injury when forward facing.