Mine is 21 weeks.. cant roll (almost though, if I help him slightly onto the side he can do the rest himself) BUT he can sit already lol.. my first was the other way around.
Mine was 3 months but I don't think specialists worry about it til closer to 8 months.
4 months!
3 Months
First baby was around 4 months, second at 3 months. I think second bubs started earlier because never swaddled him, but had my first tightly swaddled until she was trying to roll