In the absence of direct observations, demographic traits such as age and reproductive status may be modeled through proxies. We examined 35 yr of over 10,000 captures of Hawaiian green turtles (Chelonia mydas) and compared results from skeletochronology studies with mark-recapture records. For 109 turtles that were captured as juveniles and later observed nesting, we estimated maturity age first from skeletochronology-based models of age to length and second by estimating age at first capture using skeletochronology and then adding the time elapsed to first nesting. The second method involving mark-recapture gave younger and less variable age estimates. From these data we developed a scaling rule that calculates that females first bred at 23 yr (95% interval: 16.8–28.1). This result was corroborated by tag returns in the Caribbean and Hawai‘i showing that green turtles first nest at 16–20 yr. We validated this approach using life table models, successfully reconstructing four decades (1973–2012) of nesting surveys at East Island, French Frigate Shoals. We then compared our results with observed somatic growth rates, which suggest that nearshore studies may sample an atypical subset of the population that is chronically sedentary and slow growing. When exact life history traits are unknown, we recommend consulting multiple lines of evidence and independently validating proxy studies.