Data from repeated, national behavioural HIV surveillance was used to identify similarities and differences between young gay and bisexual men (YGBM) aged 16-24 (n = 1608) and older GBM aged 25-49 (n = 6266), in order to quantify risks and identify gaps in HIV prevention. Trend analyses from 2014 to 2018 are also reported. While there were similar rates of condomless anal intercourse with casual partners in both age groups, YGBM had lower rates of HIV testing and PrEP use but also fewer male partners. Our results suggest most YGBM have lower HIV risk than older GBM but a minority merit enhanced testing and prevention efforts.