I was born two and a half months early, and weighed three pounds. That was back in 1970.
I could read at the age of three.
And I started kindergarten at the age of four.
I don’t think that there are really any hard and fast rules about how long it takes to “catch up.” My parents say there was never any “catching up” for me to do and that I was always ahead of my peers.
I was always TINY, though. And that has more to do with genetics than any difficulties with being born premature.
The pediatricians never put me on a special chart to gauge my development when compared to other babies.
My grandfather was born in the early 1890s, the first of 5 sons. He was 3 pounds at birth. He said he was so small they needed to carry him around on a pillow. He never had any learning problems or deficiencies his whole life.... in fact as the son of a farmer he had lots of hard work facing him.
This fine gentleman (who always claimed to be Santa Claus’ brother) left us at age 88, having lived a full life with no health problems ever.
From the various replies it seems that it really varies from child to child.
There is all kinds of potential - that’s good.