What is teething and when do babies start teething? Teething is a process where a baby’s first teeth start breaking through the gums. Teething usually happens around the first 4 to 6 months of age, but it varies for each child anywhere between 3 to 12 months. The first set of teeth to emerge is the ones in the lower front. The upper front teeth usually emerge one or two months after the lower front teeth have come out. The last set of teeth to appear is usually the ones at the back of the mouth. And by the time the babies are 3 years old, all 20 primary teeth will have emerged.