4-6 months old. While most infants get their first teeth between 4-6 months, there is a lot of normal variability. As long as they are healthy otherwise, i wouldn;t worry about when they start teething. One thing to note is that children who get their baby teeth later also tend to get their permanent teeth later as well.
Variable. Generally 6-8 months, but there is huge variability.
Around 6 months. Most babies will get their first tooth, usually a front, lower one, around 6 months of age. They will then usually get about one a month until the front 8 teeth have erupted. There is some genetic variation, in that some families get their teeth early, some get them late. You will find that they come in on their own schedule.
Genetic schedule. If parents teethed early or late, children will follow same pattern. 1st tooth (bottom front) usually erupts about 6 months, but 2mo to 18mo is not unusual. Important that child be seen by dentist (preferably pediatric dentist for same reasons you see a pediatrician) no later than 1st b'day, whether teeth have erupted or not.