The mere presence or population density of long-lived animals such as freshwater mussels (Unionoida) does not show that the population is viable. Information on age structure may help indicate whether the population is viable, as long as samples of animals for constructing age structures are unbiased, collected using strong sampling designs and methods, and not simply casual collections of animals. We have begun to collect information on age structures of freshwater mussel populations in southeastern New York, and have found all sorts of populations – some with many young animals, some with just a few young animals, and some with no animals <50 years old. These populations clearly have different long-term prospects, and require different management approaches. In addition to identifying whether populations are viable, age structures can be used to identify the time at which conditions became unsuitable for the species, which may be useful in testing hypotheses about the causes of population declines, and to parameterize demographic models for more formal analyses of population viability. Analyses of mussel age structures are at an early stage, and would benefit from better methods for aging mussels, more data on population age structures, and demographic models that are tailored for mussels.