Background: Major depressive disorder (MDD) with comorbid anxiety is very common and is associated with worse clinical outcomes. MDD patients at different ages of onset may have different clinical features and asso-ciated factors. The aim of this study was to investigate the prevalence of anxiety and related factors in MDD patients at different ages of onset.Methods: A total of 1718 first-episode and drug-naive (FEDN) MDD patients were recruited. The cutoff point for early-adulthood onset (EAO) and mid-adulthood onset (MAO) was the first depressive episode before or after age 45 years. Clinical features (depressive, anxiety and psychiatric symptoms) and some metabolic parameters were collected.Results: There was no significant difference in the prevalence of anxiety between EAO patients (50.7 %) and MAO patients (55.7 %). For EAO patients, regression analysis showed that TSH levels, TgAb levels, and TC levels were significantly associated with anxiety. For MAO patients, regression analysis showed that anxiety was associated with HDL-c levels and impaired glucose metabolism. Furthermore, suicide attempts, psychotic symptoms, and depression severity were correlated with anxiety in both groups. Limitations: Our cross-sectional study cannot explain the causal relationship between anxiety and related factors in MDD patients at different ages of onset. Conclusions: This study revealed that the clinical characteristics and factors associated with anxiety in MDD patients differed according to age of onset, and therefore age of onset needs to be considered while treating anxiety.