When Do Babies Sit Up? 6 to 8 months of age, says Michelle Baboun, a pediatric physical therapist in Florida. At about 6 months old, most babies can sit, using their arms propped in front of them for support. On average, babies begin sitting up at around 6 to 8 months of age, says Michelle Baboun, a pediatric physical therapist in Florida. At about 6 months old, most babies can sit, using their arms propped in front of them for support.