Age at first reproduction (AFR) has been difficult to quantify in mammals, as the most commonly used methods require reproductive tracts or direct observations. However, work in several large mammal species suggests that the width of cementum light bands in teeth decline once females begin to reproduce, suggesting that teeth structures might provide a new tool to examine AFR. To determine if changes in cementum light band width could be used to calculate AFR for the northern sea otter (Enhydra lutris kenyoni), we measured cementum light band widths on sectioned premolar teeth and compared them to reproductive tracts. We classified otters as parous if any single light band was narrower than a threshold value, selected as the value that minimized error rates. At a threshold value of 0.32, we correctly identified otters as parous or nulliparous in 83% of cases (n = 92) as compared to reproductive tracts, and the AFR estimated from teeth samples (3.52 ± 0.032 yr) was not different from that determined by reproductive tract analysis (3.45 ± 0.031 yr; t-test, P > 0.05). These data support the use of cementum as an indicator of past reproduction in individual female otters, which can then be used to estimate average AFR. Given that declines in cementum width have been described for other mammal species, the same quantitative approach used here could be applied to other species.