While the average time for the appearance of the first teeth is between five and seven months of age, there is a wide normal variation of timing. The teeth might come in as early as one month of age, or they might wait until a child is almost one-and-a-half-years old. Anywhere in this range can be normal.
Generally lower teeth come in before upper teeth, and generally girls' teeth erupt earlier than those of boys (much like with everything else).
Front lower teeth usually come in first.