BACKGROUND: Individuals with inflammatory bowel disease (IBD) are at an increased risk to developing psychiatric disorders (PDs). However, the age of onset patterns for IBD-related PDs and the role of post-IBD lifestyle on the risk of subsequent PDs remain unclear. METHODS: 394,851 PD-free participants at baseline from the UK Biobank were included, of which 4408 individuals had a history of IBD (2851 ulcerative colitis [UC] and 1200 Crohn's disease [CD]). A combined lifestyle score was constructed and classified into three categories. Cox regression models were applied to evaluate the associations of age of IBD onset and lifestyle categories with PDs risk. RESULTS: Over a median follow-up of 13.6 years, 56,392 participants were diagnosed with PDs. Participants with IBD had a higher risk of psychiatric morbidity [IBD: hazard ratio (HR):1.20, 95 % confidence interval (CI): 1.11-1.29; UC: HR: 1.21, 95 % CI: 1.11-1.33; CD: HR: 1.15, 95 % CI: 1.01-1.31] compared to non-IBD participants. The risk of PDs increased with advancing age of IBD onset [</=40 years, HR: 1.16, 95 % CI: 1.05-1.29; 40-50 years, HR: 1.19, 95 % CI: 1.02-1.39; >50 years, HR: 1.26, 95 % CI: 1.11-1.42]. Subgroup analysis showed that IBD patients receiving medications did not exhibit an elevated risk of PDs, irrespective of age of onset. Additionally, adherence to a favorable lifestyle significantly reduced the risk of PDs among IBD patients (HR: 0.53, 95 % CI: 0.42-0.67). CONCLUSION: Individuals with later-onset IBD are more vulnerable to develop any PDs than younger, but adherence to a favorable post-IBD lifestyle and medications may protect against PDs.