We've always been told to go by adjusted age with development, until DS1 turned 4 and then they stopped adjusting his age.

FWIW, DS1 didn't roll until 9 months (birth age) he then crawled at 10 months and 2 weeks later he started to sit, it was just boom boom boom, once he started to realise he could move that was it!

Also, when she does start to move expect that her weight will taper off too, my boys plateaud and didn't gain much at all between 12-24 months.

At 6 months birth age, I wouldn't expect her to be doing too much atm, especially given her start in life. Enjoy her while she's little and I totally agree with the when shes ready bit, she's a premmie... they all do it when they are ready Even as they get older.... LOL.... it's always on their terms!