S.C.
My first son was born only 3 weeks early, but had intra uterine growth retardation (IUGR). There is no telling how early he was truly born because they just stop developing after a certain point. Anyway - he did not start walking until he was 18 months old. The Drs. told me that with a premature baby, the neurological development is much slower and that until that part of the brain is fully developed, they wont walk. I tried to help him build the muscles by putting him in his walker at the lowest setting so that he would have to stand up and walk to move around. I also bought a jumper and put it pretty low so that he could learn balance of standing up in a place where he felt comfortable and knew he would not fall if he lost his balance.
It is difficult to have a child that seems to be slow in the stages of life, but children like ours do catch up! My son is 4 1/2 now, he is still small for his age, but not by much. He is also reading at a pace that most boys his age don't come close to - so don't worry!! When he is ready to walk - he will.
Good luck!