Age at which cattle become faecal culture positive for Mycobacterium avium subsp. paratuberculosis (Map) can be used as a proxy parameter for age at becoming infectious, which is an important parameter in the control of Map in cattle herds. To investigate the age at which cattle become faecal culture positive, survival analysis methods were applied to faecal culture results of 37151 Dutch dairy cattle in 373 herds between 1996 and 2002. A Weibull proportional hazards model was employed to study effects of risk factors such as apparent prevalence within the herd, dairy breed and herd size.The analyses showed that the hazard of becoming faecal culture positive increased with the age of cattle. At a higher within-herd apparent prevalence, cattle became faecal culture positive at younger age. In high prevalence herds (apparent prevalence ≥ 0.10), an estimated 5.4% to 13.7% of cattle became culture positive before 2 years of age, depending on breed and herd size.Our findings indicate that a considerable proportion of young stock was shedding Map. Therefore, infectious young stock should be a major concern in the control of paratuberculosis, especially in high prevalence herds.