OBJECTIVE: To describe recent trends among men who have sex with men (MSM) in age at diagnosis of HIV in Victoria. DESIGN AND SETTING: Analysis of Victorian HIV surveillance data from (i) passive surveillance (2000-2009) and (ii) the Victorian Primary Care Network for Sentinel Surveillance (VPCNSS) (2006-2009). Age-trend comparisons were made using syphilis and gonorrhoea enhanced surveillance. MAIN OUTCOME MEASURES: HIV diagnoses, HIV testing and behavioural indicators by year and age group among MSM. RESULTS: Following a period of sustained increase between 2000 and 2007, the median age at HIV diagnosis among MSM declined significantly, from 38.8 years in 2007 to 35.3 years in 2008 (P=0.023), remaining at 35.9 years in 2009. Between 2007 and 2008, the median age of syphilis and gonorrhoea notifications also declined, from 40.6 to 36.0 years and from 32.3 to 29.3 years, respectively. The median age of HIV testing among MSM in the VPCNSS population remained constant between 2006 and 2009, at 33.0 years. Compared with older MSM, those aged less than 35 years were more likely to have never previously been tested for HIV (relative risk [RR], 1.36 [95% CI, 1.30-1.41]); to not know the HIV status of their regular partner (RR, 1.11 [95% CI, 1.01-1.21]); and to report inconsistent condom use with casual partners (RR, 1.07 [95% CI, 1.01-1.14]) and regular partners (RR, 1.07 [95% CI, 1.00-1.14]). CONCLUSIONS: Younger MSM in Victoria may be at increasing risk of HIV infection. Enhanced methods of monitoring HIV and sexually transmitted infection transmission in younger MSM are needed, as well as prevention messages to target this group, who may not fully understand their HIV risk.