A baby's teeth erupt as early as 3 months! My nephew was sort of a late bloomer. His started to erupt at about 5-6 months. There's a special kind of toothbrush for babies available in drug stores/supermarkets/baby stores. Brush their teeth twice a day with a smear of toothpaste. Increase this to the size of a pea when your kid turns 2.
Other ways to care for their teeth would be not to send them to bed with a bottle and to transition from the bottle to a cup sometime after their 1st birthday.
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