I have a granddaughter born at 30weeks.She was very ill when first born, did not ''do anything'' until 6 months old, in fact her parents were told she had possible brain damage.
She suddenly started to smile and take notice of everything, although slow at sitting upas she could not support her head.
Then just after her first birthday she started to pull herself up and walk, began to make words and do everything she should be doing for her age.
Two of my other grandchildren, born full term never started walking until 22 months old, one walking on her knees from 12 months.Their father, my DS2 was the same.
My premature granddaughter is nearly 26 now and no problems.