Forum crashing here because my kids are older.
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Our ds sat independently at 9 months, which is the tail end of typical. Dd was 8 months. I think the age to be 'worried' is 10+ months. The other thing is that it can go really quickly. I've got pictures of ds at 8 months lying under the Christmas tree, and a 3 weeks later, he was getting into a sit and sitting very well.
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Ds went from sitting, to crawling, to pulling up within a matter of weeks. He's always been the kind of kid who won't try something until he knows he can do it. He turned out to have a bit of motor planning issue that was treated when he got occupational therapy for sensory issues. However, our whole family was 'late' to sit (dd was 8 months, which ain't early) and most of us are fine physically. OK, we're not elite athletes, but we can do what we need to. And just because your 7 mo old isn't sitting now, means very little about future development.