Lacking experience represents a constraint on solitary breeding, which may be overcome by joining groups of more experienced breeders. Also, biological market theory predicts preference for partnerships with conspecifics of the highest value. We examined the formation of cooperative brood-rearing coalitions in facultatively social eider females, Somateria mollissima. Our aim was to elucidate the hitherto neglected role of age on female group size and condition differences within groups, and to explore whether older females represent attractive coalition partners. Based on previous work, we hypothesized that older breeders are found in smaller groups, and that the attraction of older partners would override the predicted negative effect of differences in body condition on female group size. We found that older females occurred in smaller groups and that the negative relationship between condition differences and group size became less steep with increasing age of the oldest group member. We also found that the maximum body condition found in a brood-rearing coalition increased with the age of the oldest female, despite a sharp decline in the probability of the oldest individual being the one in best condition. These results demonstrate the importance of age in the formation of cooperative brood-rearing coalitions and the increase in female quality with age. The experience of older females is a valuable commodity, which attracts younger prospective coalition partners in good condition regardless of body condition differences. Our results illustrate the general principle that grouping decisions cannot be understood by viewing partner choice criteria in isolation.