Lower curative surgery, radiotherapy, and chemotherapy rates in older lung cancer patients (age 75 years and older) does not necessarily imply under-treatment due to contraindications arising from these patients’ higher prevalence of advanced disease, comorbidities, and frailty. The study explores a definition of under-treatment as the proportion of the survival deficit in older patients that is due to their lower curative treatment rates adjusted for clinical and demographic factors. The study adopts a causal inference statistical approach, firstly, requiring a Directed Acyclical Graph (DAG) to clarify the appropriate estimator required, and secondly, proceeds to employ this estimator with real data using the g-computation method and data imputation. Sensitivity analysis and simpler Structural Equation Models are used to elucidate the main causes of under-treatment. Data on all lung cancer patients diagnosed between 2010-2014 from the Northern Ireland Cancer Registry (NICR) was linked to hospital episode and multidisciplinary team meeting datasets, allowing the analysis to consider patients’ disease stage, comorbidities, performance status, pulmonary function (FEV1/FVC ratio) at time of diagnosis. With an ever-growing cohort of older lung cancer patients entering an underfunded health service in Northern Ireland, population-based measures of undertreatment will inform how best the health service meets the needs of this patient population.