Despite a recent fall in the incidence of HIV within the UK, men who have sex with men (MSM) continue to be disproportionately affected. As biomedical prevention technologies including pre-exposure prophylaxis are increasingly taken up to reduce transmission, the role of HIV testing has become central to the management of risk. Against a background of lower testing rates among older MSM, this study aimed to identify age-related factors influencing recent (≤12 months) HIV testing. Cross-sectional subpopulation data from an online survey of sexually active MSM in the Celtic nations-Scotland, Wales, Northern Ireland and Ireland (n=2436)-were analysed to compare demographic, behavioural and sociocultural factors influencing HIV testing between MSM aged 16-25 (n=447), 26-45 (n=1092) and ≥46 (n=897). Multivariate logistic regression demonstrated that for men aged ≥46, not identifying as gay (OR 0.62, CI 0.41 to 0.95), location (Wales) (OR 0.49, CI 0.32 to 0.76) and scoring higher on the personalised Stigma Scale (OR 0.97, CI 0.94 to 1.00) significantly reduced the odds for HIV testing in the preceding year. Men aged 26-45 who did not identify as gay (OR 0.61, CI 0.41 to 0.92) were also significantly less likely to have recently tested for HIV. For men aged 16-25, not having a degree (OR 0.48, CI 0.29 to 0.79), location (Republic of Ireland) (OR 0.55, CI 0.30 to 1.00) and scoring higher on emotional competence (OR 0.57, CI 0.42 to 0.77) were also significantly associated with not having recently tested for HIV. Key differences in age-related factors influencing HIV testing suggest health improvement interventions should accommodate the wide diversities among MSM populations across the life course. Future research should seek to identify barriers and enablers to HIV testing among the oldest and youngest MSM, with specific focus on education and stigma. © Author(s) (or their employer(s)) 2019. No commercial re-use. See rights and permissions. Published by BMJ.