There are key differences between primary and permanent teeth. [1] Primary teeth, also called baby teeth or milk teeth, are the first set of teeth that start erupting around 6 months of age and are replaced by permanent teeth. [2] Primary teeth have thinner enamel and are lighter in color, while permanent teeth have thicker enamel and are darker in color. [3] Primary teeth are eventually replaced by 32 permanent teeth that develop larger and stronger over time from ages 6 to 21 years.