Not to derail the thread, but I'm in a group of about 15 mums, all with babies the same age and it was pretty much 50/50 the babies who could and couldn't sit up by 6 months. And even with the ones who could sit up (mine included), most were a little bit wobbly still, and an insert in the high chair made them more secure. And for the ones who couldn't yet sit up, it definitely wasn't through a lack of tummy time or anything like that because it was something we definitely all did and talked about a lot!