In many species, young males are the dispersers, leaving their natal area after weaning to establish a breeding area of their own. As young males disperse, however, they are bound to encounter unfamiliar adult males with established territories. Such interactions between an adult male and a young male may always be agonistic. Alternatively, there may be an age threshold below which aggression is not elicited and above which the adult male is aggressive toward the juvenile male. To test these two alternative hypotheses, we paired 47 young Syrian hamster (Mesocricetus auratus) males ranging from 24 to 65 days of age with 47 adult male hamsters and measured aggressive and investigatory behavior for 5 min. We observed no aggression by the adult toward young males between 24 and 47 days of age or toward the single male that was 49 days of age. Young males that were 50 days of age or older, however, elicited significant levels of aggression from the adults. These results indicate that in Syrian hamsters, young males are less vulnerable to adult aggression up to an age threshold and are more vulnerable to adult aggression beyond that threshold. This pattern may facilitate the establishment of territories by dispersing young males below that age threshold. Aggr. Behav. 37:91–97, 2011. © 2010 Wiley-Liss, Inc.