Elephant

Unlike most mammals, which grow baby teeth and then replace them with a permanent set of adult teeth, elephants have cycles of tooth rotation throughout their entire lives. The tusks have milk precursors, which fall out quickly and the adult tusks are in place by one year of age, but the chewing teeth are replaced five or, very rarely, six times in an elephant's lifetime.
Only four chewing teeth (premolars and/or molars), one on each side of each jaw, are in primary use at any given time (or two, as one replaces the other at each location). Adult teeth do not replace milk teeth by emerging from the jaws vertically as human teeth do. Instead, new teeth grow in at the back of the mouth, pushing older teeth toward the front, where the latter break off in pieces until they are gone. In African elephants, the first two sets of chewing teeth (premolars) are in place when an elephant is born. The first chewing tooth on each side in each jaw falls out when the elephant is about two years old. The second set of chewing teeth falls out when the elephant is about six years old. The third set is lost at 13 to 15 years of age, and set four lasts to approximately 28 years of age. The fifth set of chewing teeth (molars) lasts until the elephant is in its early 40s. The sixth (and usually final) set must last the elephant the rest of its life. If an elephant lives to more than 60 years of age, the last set of molars is worn to stumps, and it can no longer feed properly. Moss reports a female elephant in its sixties whose final set of molars were worn smooth and about one-quarter of their original size and who survived "with extra chewing and longer feeding bouts." Abscesses of chewing teeth, as well as of tusks and jaws, are common in elephants, and may lead to premature death.
Tusks in the lower jaw are also second incisors. These grew out large in Deinotherium and some mastodons, but in modern elephants they disappear early without erupting.