CONTEXT: Non-functioning pancreatic neuroendocrine tumours (NF-pNETs) are highly prevalent and constitute an important cause of mortality in patients with multiple endocrine neoplasia type 1 (MEN1). Still, the optimal age to initiate screening for pNETs is under debate. OBJECTIVE: To assess the age of occurrence of clinically relevant NF-pNETs in young MEN1 patients. PATIENTS AND SETTING: Pancreatic imaging data of MEN1 patients were retrieved from the DutchMEN Study Group database. DESIGN: Interval-censored survival methods were used to describe age-related penetrance, compare survival curves, and develop a parametric model for estimating the risk of having clinically relevant NF-pNET at various ages. INTERVENTION(S): Not applicable. MAIN OUTCOME MEASURE(S): The primary objective was to assess age at occurrence of clinically relevant NF-pNET (size ≥20 mm or rapid growth); secondary objectives were the age at occurrence of NF-pNET of any size and pNET-associated metastasized disease. RESULTS: Five of 350 patients developed clinically relevant NF-pNETs before age 18, two of which subsequently developed lymph node metastases. No differences in clinically relevant NF-pNET-free survival were found for sex, timeframe, and type of MEN1 diagnosis or genotype. The estimated ages (median, 95% CI) at a 1%, 2.5% and 5% risk of having developed a clinically relevant tumour are 9.5 (6.5 - 12.7), 13.5 (10.2 - 16.9) and 17.8 years (14.3 - 21.4) respectively. CONCLUSION: Analyses from this population-based cohort indicate that start of surveillance for NF-pNETs with pancreatic imaging at age 13-14 is justified. The psychological and medical burden of screening at a young age should be considered.