If a child isn't able to sit up unassisted by 9 months old, they may be experiencing a gross motor skills delay. The CDC recommends parents talk to a pediatrician if their child doesn't meet this important milestone.
Reasons why a baby can’t sit up may include:
Developmental disorders like cerebral palsy or autism spectrum disorder
Genetic disorders like Down syndrome
Health complications caused by a premature birth
Other issues impacting muscle development and function
Physical and occupational therapy can help many children improve their gross motor skills.