At what age do kids "typically" go from lying down (flat on their back) to sitting up - like doing a sit-up? D is 18 months and still not able to do that. He's also not walking unassisted yet. If he's still not walking at his next appointment (which, unfortunately, won't be for another month since we're between insurances at the moment), we'll talk about Early Intervention. Just wondering if not sitting up is something we need to point out and discuss, as well. Thanks!