Learning to stand starts when the baby first takes weigh on the legs when he/she is held upright at about 6-7 months. With practice and repeated attempts babies develop the strength and coordination to pull themselves up into standing from a sitting or kneeling position.
They learn to stand supporting with two, and then with one hand, to reach in all directions, and to bend the knees to sit down again.
Typically developing babies spend many, many hours working at achieving each new step along the pathway to independent standing and then walking.
Premature babies, those with low muscle tone, joint hypermobility, developmental coordination disorder and autism may need extra help and encouragement to achieve the strength, flexibility and coordination for learning to sit with good balance.