There is growing interest, within the social sciences, in understanding self-quantification and how it affects health practices in contemporary society. There is, however, less research on how ageing and health measurement relate, even though this relationship has become more pertinent with the growing availability of services and devices offering biological, personalised age measurements, from simple on-line questionnaires to telomere length quantification. Little is known about who uses these devices, why they use them and the socio-technical implications of such uses. To explore these issues, we conducted semi-structured interviews and focus groups with users of measurements of biological age in Denmark. We found that participants engage with the measurements with a degree of scepticism regarding their technical validity, reliability and sensitivity. Rather than seeking an exact biological quantification, participants use measurements as a pragmatic, rough indication of individual health. We develop a conceptual model to understand participants’ engagement with biological age measurements, which suggests that, instead of a substitution of chronological age for biological age, users gauge the difference between the two to qualify their present and future individual trajectory in a lay model of the relationship between functional capacity and age.