By which age should a child be able to sit up?

A child typically begins to sit up independently around 6 months of age. At this stage of development, the child has usually gained sufficient trunk control and muscle strength to maintain a seated position without assistance. Prior to 6 months, infants may be able to sit with support or while propped but not yet independently. By 6 months, most babies can sit upright for short periods, which is considered a significant milestone in their physical development.

As children continue to grow, their ability to sit up becomes more stable, and by 9 months, they are generally capable of sitting well without support. However, the specific milestone for sitting independently occurs around the 6-month mark, making this age the correct answer.