Age estimation of individuals is often an integral part of species management research, and a number of age-estimation techniques are commonly employed. Often, the error in these techniques is not quantified or accounted for in other analyses, particularly in growth curve models used to describe physiological responses to environment and human impacts. Also, noninvasive, quick, and inexpensive methods to estimate age are needed. This research aims to provide two Bayesian methods to (i) incorporate age uncertainty into an age–length Schnute growth model and (ii) produce a method from the growth model to estimate age from length. The methods are then employed for Florida manatee ( Trichechus manatus ) carcasses. After quantifying the uncertainty in the aging technique (counts of ear bone growth layers), we fit age–length data to the Schnute growth model separately by sex and season. Independent prior information about population age structure and the results of the Schnute model are then combined to estimate age from length. Results describing the age–length relationship agree with our understanding of manatee biology. The new methods allow us to estimate age, with quantified uncertainty, for 98% of collected carcasses: 36% from ear bones, 62% from length.