Babies usually start teething from 4 month old, but can be lot earlier or lot later. Some babies are born with teeth. Look for the sign of teething: lots of dribbling, furious chewing and biting down on anything put near his mouth, awful rash, lots dirty runny nappies and etc.

My little girl was around 10 weeks when she started teething and she got her first teeth at four months but was definitely teething for a while. My friends daughter was teething when she was born and had her first tooth come through when she was a week old, so any age is possible.