Much research is conducted to evaluate digital-based solutions for health-care services, but little is known about how such evaluations acknowledge diversity in later life. This study helps fill this gap and analyses participation in the evaluation of a web-based mobile phone system for monitoring the post-operative progress of patients after day surgery. Participation is conceptualised as resulting from three processes: pre-screening, recruitment and self-selection. Based on field information and survey data, this study models (a) the (non-)participation in a sample of 498 individuals aged 60 and older that includes non-screened, non-recruited, decliners and participants in the evaluation, and (b) the individual decision to participate in a sample of 210 individuals aged 60 and older who were invited to take part in the evaluation. Increasing age enhances the likelihood of not being screened, not being recruited or declining the invitation. Those not recruited were most often ineligible because of technology-related barriers. Decliners and participants differed by age, gender, job, health status, digital skills, but not by social participation. Results suggest that highly specific groups of older people are more likely to be involved than others. Old-age diversity is not properly represented in digital health research, with implications for the inclusivity of new digital health technologies. This has implications for increased risks of old-age exclusion and exacerbation of social and digital inequalities in ageing societies.