8-9 kgs is the minumum weight. Most car seats will safely hold your child rearward facing until they are 12 kgs. Check the instruction booklet, not the seat or the box. The seat and the box will usually have the minimum weight listed on it.
My DD is 9 kgs and almost 7 months old and still facing backwards. She is long enough to touch the back of the seat but is not at all uncomfortable. Ive attatched a toy to the seat so that when she kicks it makes a noise, keeps her very entertained.
Beth, 6.5kgs is too light. You wont do her any harm by leaving her for another 2 months. The longer they are rearward facing the safer they are.
Karen, DS1 31/8/03, DD 12/3/07, DS2 16/12/09