Hair follicles start developing in the third month of pregnancy. By the time a child is born, on average 100,000 hair follicles have developed on its head, equivalent to 1,000 follicles per square centimetre of scalp. Follicle density declines from birth onwards, in line with the growth of a child's scull. As an adult, the total number of follicles declines to some 500 follicles per square centimetre by the time a person reaches the age of 25, to 150-250 between 30 and 50, further declining as we get even older.