James L. Bodkin, Jack A. Ames, Ronald J. Jameson, Ancel M. Johnson, Gary M. Matson, Estimating Age of Sea Otters with Cementum Layers in the First Premolar, The Journal of Wildlife Management, Vol. 61, No. 3 (Jul., 1997), pp. 967-973
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Bodkin et al. (1997) studied this question.