A baby may begin sitting up with some help by 4–6 months of age, and at 6 months, they may not need assistance. By 9 months, a baby should be able to get into a sitting position without any support. It is important to keep in mind, however, that every baby develops at a different rate.

It was at around the six month mark for my daughter.

I think it was around the 6 month mark for both mine.

About 6 months. Some are a little earlier or later

Around 6 months but some take longer.