Childrens' Car Seats

 Looking at different sites about when you need to use a childrens' car seat I am slightly confused as whether the age and height restriction is an either choice or whether they have to be both.  Both my Children are under 12 but over the height restriction - I can't see the logic in them having to use booster seats.  Does anyone know for sure whether you have to be both over the age and height or just the height is acceptable?

 Although  I haver seen a car in use in Spain these are the rulkes.

the booster seat is so the seat belt doesn't go cross their heads

What are the requirements?

Up to 9 months and up to 10 kg, a carrycot must be used laid across the rear seat and secured with the car's safety belt or any specially approved attachment.

From 10kg to 13kg and up to 2 years of age, a child's car seat fitted in the rear with the child facing rearwards.

From 9kg to 18kg and from 9 months to 3 years old, child seats facing frontwards or rearwards , fitted in the rear of the car and secured as detailed in the seats instructions.

From 15kg to 25kg and 3 to 6 years of age, use a seat adaptor (elevator) to lift the child up to fit comfortably with, if necessary, a CE-EU approved seat belt adaptor.

From 22kg to 36kg and 6 to 12 years of age, use a seat elevator so that the child is comfortably protected using the seat belts.

Children up to the age of 12 are not allowed to sit in the front seat.