In many species, the genetic quality of gametes is likely to decline with age of the male. Some form of discrimination against mating with old males, or against fertilization by their sperm, can be expected to evolve. We investigated the effect of male age on the probability of mating and on sperm competitiveness in the bulb mite, Rhizoglyphus robini. There was no significant difference between male age classes in the probability of mating with virgin females, but young males were more likely to mate with already mated females. Furthermore, young males won sperm competition against older males. This implies that polyandry may be an effective way of avoiding fertilization by the sperm of older males. It has also been suggested that polyandry insures females against the infertility of old males. Our results do not support this hypothesis: matings with older males did not result in female infertility significantly more often than matings with young males.