Babies born premature develop as they would in the womb until their due date, so they will achieve reach various milestones based on when they were due. Even measuring from the date the baby was due, a baby born premature is likely to be slightly slower at reaching some physical milestones such as sitting up, crawling or walking. This is because before birth babies bounce against the walls of the uterus, which strengthens their muscles. A baby born premature is not able to get this exercise. It is common, in the UK at least, to refer to babies as having an actual age and a ‘corrected age’. My daughter who was born 3 months early walked at 18 months, which would be a corrected age of 15 months.