evidence: The relative contribution of different age groups to undiagnosed HIV infection varies between different exposure groups. In GBMSM, those aged 15 to 34 years old have dropped from accounting for 58% (95% CrI 34% to 78%) of those undiagnosed in 2019, to 49% (95% CrI 27% to 70%) in 2021. As a result, most GBMSM living with undiagnosed HIV are now estimated to be over the age of 34 years (51%, 95% CrI 30% to 73%). In contrast, among heterosexual men living with undiagnosed HIV, the largest age group was those aged between 35 and 44 years between 2019 (36% of those undiagnosed, 95% CrI 23% to 47%) and 2021 (37% of those undiagnosed, 95% CrI 26% to 47%). During this period, the estimated proportion of undiagnosed heterosexual men in the 15 to 34 age group slightly reduced (26% in 2019, 95% CrI 11% to 47%, and 23% in 2021, 95% CrI 10% to 42%; probability of decline 59%).