Kittens, like human babies, are born without teeth. Only the gum surface can be seen. This allows them to nurse without hurting the mother. When they reach 25 to 30 days old their baby teeth start to break through the gums, which is referred to as "erupting." Kittens have a total of 26 deciduous teeth by the time they reach 45 days of age. As time goes by, these teeth are replaced by permanent teeth. The feline tooth fairy works somewhat faster than her human counterpart - most breeds show permanent teeth at 6 months of age.
To learn more, read How Old Is This Kitten.