Knowing the age of individuals in a population is crucial to devise appropriate conservation strategies, especially on endangered long-lived species such as sea turtles. Sea turtles do not have external morphologic characters that indicate their age; therefore, age has been estimated by indirect methods such as biometric measurements or skeletochronological analysis. Previous skeletochronological studies have determined that sea turtle age can be estimated with skeletal growth marks (GMs) produced by annual seasonality. This study focused on the skeletochronological analysis of known-age loggerheads reared under seminatural conditions, showing that sea turtles kept under natural photoperiod and seawater temperature seasonality and fed periodically, did not present visible GMs. However, the animals suffering health problems affecting their growth did exhibit GMs. This suggests that skeletochronological studies in sea turtles must be taken with caution at least at early life stages and for the northeast Atlantic loggerhead population. Additionally, this study showed that seawater temperature seasonality strongly influences sea turtles’ growth rates.