Some babies have a visible tooth at birth. This is called a "dentes natales" and is a tooth-like rudiment. It usually has to be removed because it interferes with breastfeeding.
The eruption of the first tooth varies greatly from person to person. Most babies get their first milk teeth by the time they are six months old. Before this time, the milk teeth are already fixed in the jaw. The lower central incisors come through first.
By the age of about one year, the upper and lower incisors are visible in the mouth. Later the molars erupt and then the milk teeth. At around 3.5 years of age, the milk dentition is fully formed. The milk dentition consists of 20 teeth.
The first milk teeth are much smaller than the permanent teeth. Their enamel is thinner and less mineralised. This is why cavities form more quickly. Milk teeth wear down more quickly than permanent teeth.