Age limits, minimum and maximum, and both explicit and ‘covert’, are still used in the National Health Service to determine access to a range of health interventions, including infertility services and cancer screening and treatment. Evidence suggests that chronological age is used as a proxy for a host of characteristics in determining access to healthcare: as a proxy for the capacity of an individual to benefit from an intervention; for the type of harm that may result from an intervention; for the likelihood of such benefit or harm occurring; and, in some cases, for other indicators used to determine what may be in the patient’s interest. Age is used as a proxy in this way in making decisions about both individual patients and wider populations; it may be used where no better ‘marker’ for the relevant characteristic exists or – for reasons including cost, practicality or fairness – in preference to other available markers. This article reviews the justifications for using age in this way in the context of the existing legal framework on age discrimination in the provision of public services.