Adjusted age for development?

Ge

We had our twins preemie at 34 weeks via emergency section, they spent a couple of weeks in neonatal unit before coming home and are currently 9 weeks and doing really well and gaining weight nicely.

I was wondering about how the prematurity affects development. I know they were 6 weeks early so effectively their adjusted age is only 3 weeks, but is this for all things development wise or will they catch up on some things? Have noticed we’re already a little behind on some things like smiling etc bless them, so was just curious about what to expect 🥰