I think I've heard that the rule of thumb is to turn them at 12 kilos or 12 months ... whichever comes first.

But DD is 12 months and 9kg and I honestly don't see the problem leaving her as she is which is rear-facing. We rarely use the car so I'm not quite sure what she does with her legs and she's quite tall for her age (about 80th percentile)!

I did put her in a friend's forward-facing car seat about a month ago and she thought all her Christmases had come at once, she thought it was great. But I'd rather keep her rear-facing if it's safer.