Abstract Purpose Capability wellbeing measures, such as the ICECAP measures, have been proposed for use in economic evaluations to capture broader outcomes of health and care interventions. The ICECAP measures have been developed to reflect capabilities at different stages of life. Some patient groups include patients of different ages and at different stages of life, so it is not always apparent which ICECAP measure is most relevant. This study explores the impact of age and life stage on completion, where both ICECAP-A and ICECAP-O were completed by the same patient. Methods A think-aloud study, and an associated semi-structured interview were conducted with people receiving kidney care as a renal outpatient, kidney transplant outpatient, or through receiving facility-based haemodialysis. Qualitative analysis focused on (1) differences in responses across measures by individuals, where attributes had conceptual overlap, (2) key factors in self-reported capability levels, and (3) measure preference. Results Thirty participants were included in the study, with a mix of older and younger adults. Attributes with similar wording across measures produced similar responses compared to attributes where wording differed. Age and health were key factors for self-reported capability levels. ICECAP-A was slightly preferred overall, including by older adults. Conclusion This study suggests use of ICECAP-A in patients with certain chronic health conditions that include a mix of adults across the life course. This study highlights the importance of considering the stage of life when using capability measures and in economic evaluations of health and care interventions more generally.