This study examined age trends in anxious and depressive symptoms, from older adolescence to old age, and explored the association between anxious and depressive symptoms with gender, ASD severity, and socio-economic factors. Two hundred and fifty-five individuals with ASD (151 males, Mage = 33.52 years, SDage = 14.98) took part. More than one-third of participants reported clinically significant anxiety (38.4%) or depression (38%). A slight trend for an increase in the severity of both anxiety and depression from adolescence to middle adulthood, and then a slight decline in older adulthood was found. Female gender and higher ASD severity predicted more anxiety and depression symptoms. Our findings emphasise the need to provide timely assessment and treatment of anxiety and depression in ASD.