In long-lived snakes estimates of survival rates in the immature age classes are notoriously difficult to obtain because the small, secretive juveniles are rarely caught in field studies. Hence, it is assumed that in many species juveniles suffer high mortality. An alternative view holds that the youngest life stages are so elusive as if they “disappear” temporarily from the population. We conducted a long-term (2000-2016) mark-recapture study in a large population of European adders and obtained demographic data for large numbers of immature and newborn snakes. Estimates obtained by the Cormack-Jolly-Seber (CJS) method revealed dramatic age-related differences in yearly capture probabilities: they were much lower in the immature classes than in the adults. Concurrently , we found no evidence for age-dependent differences in survival rates. Hence, our inability to capture large numbers of immature snakes should therefore be attributed to their low detection probabilities, not because they would suffer high post-natal mortality. At least three traits contribute to the poor detectability of the immature snakes: (1) their small body size, (2) their lower thermal requirements, and (3) their non-permanent emigration to the “summer” or foraging habitats, which possess a greater food supply than the “winter” habitats.