Premmie babies don't actually "catch up" They just grow to a stage where you can't tell the difference anymore.

So, for a baby like Sam, born at 36 weeks, that point comes quite early, because it's towards the end of the first year where you start to not really notice a couple of weeks. A baby born at 30 weeks might appear "caught up" a bit before two. Working in childcare, I have had instances where we have taken adjusted age into account right up to 4 years, for a little guy born at 24 weeks, because the difference is enormous for quite a long time.

The same sort of applies for reaching milestones. A baby born at 24 weeks couldn't possibly be expected to reach, say a walking milestone at the same time as a full-termer, but a baby born at 35 weeks probably would.

It completely depends on the child.