Background: Older patients are major users of pharmacotherapy due to a higher incidence of health issues. However, there is evidence of age-biased prescribing, leading to over- or underprescribing of medication, and suboptimal clinical outcomes. Although many guidelines provide cautionary statements about the use of medicines in older patients, they fail to identify what this means in practice. There is no accepted definition of an older adult that appropriately characterises this patient group. As a result, there is potential for physicians to have variable interpretations of individuals within this patient population, leading to potential inconsistencies when making pharmacotherapeutic decisions. Objective: The aim of this study was to explore how Australian medical physicians practically defined an older adult patient in the context of providing pharmacotherapeutic care to this population. Methods: This was a two-stage study comprising a scenario-based questionnaire (quantitative phase) and semi-structured individual interviews (qualitative phase) with Australian physicians. Qualitative data was thematically analysed and manual inductive coding was used to generate core themes. Results: A total of 15 physicians participated in the study. Overall, in regard to providing care to their older patients, the three key themes that emerged from physicians’ discussions were (1) using a number-based versus health status-based definition of an older patient; (2) patient ‘red flags’ influence prescribing decisions; and (3) lack of guideline support in prescribing for older patients. Most physicians ultimately defined older adult patients using a number-based description (i.e. age between 65 and 90 years) because they felt they needed some sort of ‘cut-off’ point to guide their decision making. However, in assessing an older patient, physicians considered a multitude of patient factors as influencers of their decision making during prescribing, including comorbidities, cognitive function, frailty, polypharmacy, etc., and did not solely focus on the patient’s age. Conclusion: Physicians describe the complexity of decision making for older adult patients, and how this is influenced by a diverse range of factors, yet ultimately simplify the process by defaulting to number-based (age in years) guidelines and procedures.