Babies typically start to sit up with some support around 4 to 6 months of age. They begin by using their arms to balance themselves (tripod sitting) and gradually develop enough strength and balance to sit independently. Most babies can sit up well without support between 7 to 9 months, with some mastering this skill closer to 8 or 9 months. The ability to sit up independently indicates increased muscular strength and coordination, and often precedes other milestones like crawling and standing.
