The burden attributable to late-life Generalized Anxiety Disorder (GAD) is set to increase alongside the ageing population. Yet, the psychogeriatric workforce is limited in size and few studies have focused on the treatment of GAD among older patients. Internet-delivered cognitive behavioral therapy (iCBT) could be integrated into existing infrastructure and scale the psychogeriatric workforce, if shown to be effective across the adult lifespan. This study examined age-related differences in presentation, adherence and effects of iCBT for GAD among patients in routine clinical care (N = 942; 18–29 years (n = 267); 30–39 years (n = 260); 40–49 years (n = 180); 50–59 years (n = 124); and 60+ years (n = 111)). Compared to younger patients, older patients were less likely to present with probable GAD and MDD diagnoses (<60 years vs. 60+ years) and more likely to complete treatment (<50 years vs. 50+ years). iCBT produced moderate to large effect size reductions in symptom severity, distress and impairment independent of age. iCBT is therefore an effective treatment for GAD across the adult lifespan, which can be delivered in routine clinical care. Continuing to integrate iCBT into existing services has the potential to improve the capacity of the existing workforce to manage those seeking help for GAD particularly as the population ages.