Policies ensuring that research data are available on public archives are increasingly being implemented at the government, funding agency, and journal level. These policies are predicated on the idea that authors are poor stewards of their data, particularly over the long term, and indeed many studies have found that authors are often unable or unwilling to share their data. However, there are no systematic estimates of how the availability of research data changes with time since publication. We therefore requested datasets from a relatively homogenous set of 516 articles published between 2 and 22 years ago, and found that availability of the data was strongly affected by article age. For papers where the authors gave the status of their data, the odds of a dataset being extant fell by 17% per year. In addition, the odds that we could find a working email address for the first, last or corresponding author fell by 7% per year. Our results reinforce the notion that, in the long term, research data cannot be reliably preserved by individual researchers, and further demonstrate the urgent need for policies mandating data sharing via public archives.