In mouflon (Ovis gmelini musimon × Ovis sp.) populations, most age-related ecological studies have used morphological characteristics to determine age categories in the field although the validity of this approach remains largely untested. We estimated error rates in age determination from observations of known-age mouflon in southern France. Based on repeated observations of 163 animals, we estimated the sex-, age- and time-variations in error rates. We showed that the age-related pattern of error was the same for both sexes and was not time dependent. Male (7.4% [6.7; 11.1] _CI95%) and female (6.7% [3.2; 12.9] _CI95%) lambs had a low and similar probability (P = 0.61) of error, whereas older mouflon, and more females (53.2% [43.2; 62.5] _CI95%) than males (27.1% [19.2; 34.5] _CI95%, P ≤ 0.001), had a high probability to be misclassified. Over and above the skill of the observers, the morphological criteria used to discriminate age categories probably account for these high and sex-specific error rates. To correct these errors, we recommend that: 1) criteria used in the field be tested on a sample of known-age animals from the studied population; or 2) field aging be restricted to well-defined age categories such as lambs, ewes, and rams.