Can babies stary teething before six months?

Yes. Yes they can, but it is less common than older ages.
Teething. The average age of teething onset is 4 to 7 months. However, some newborns are born with soft "natal teeth" which are often loose and must be removed to avoid choking hazard. Drooling may occur for weeks prior to the onset of teething. Very swollen gums occur then sharp edge of tooth can be felt. It can take up to 2-3 weeks for tooth to fully erupt.
Teething. Typically infants will have their first tooth come in between 6-10 months of age. Some newborns are born with natal teeth, thus it is possible for infants to have early teething. It must be kept in mind that symptoms such as fussiness and drooling which are common signs of teething can also be signs of other things, such as infection.
Yes. Although the milk teeth develop when the baby is growing in the womb, the teeth only start to grow throughout the gums when the baby is 6-9 months old (although it can be before or after these ages)
Yes. Most infants start teething between 4-8 months of age. There is a very wide range of what is "normal". I have seen several infants born with bottom front teeth already and many who didn't get their first tooth till 9-14 months old. In almost all cases it just works it own way out as the child grows.
Yes. The typical range begins at 6 months, but that doesnt mean that some children do not start earlier. As long as the sequence of eruption follows the correct pattern, in most cases early or late teething is not a reason for concern.