We should have one rule very clear in our minds: children can only wear a seat belt when it properly fits their body, and when no sensitive part of their body is put in danger, such as their neck. Until this point, they should use a child restraint system suitable for their height and weight.
We should not be in any rush. Although the regulations of a country might be more restrictive, a child car seat should always be used whenever necessary. For example, in Spain a child must travel in a CRS until they reach a height of 135 cm. However, at the European level the height of 150 cm is a more widespread cut-off point.