BACKGROUND: Limited evidence suggests that younger people who inject drugs (PWID) engage in high-risk injecting behaviours. This study aims to better understand the relationships between age and risky injecting behaviours. METHODS: Data were taken from 11 years of a repeat cross-sectional study of sentinel samples of regular PWID (The Australian Illicit Drug Reporting System, 2001-2011). Multivariable Poisson regression was used to explore the relationship between age and four outcomes of interest: last drug injection occurred in public, receptive needle sharing (past month), experiencing injecting-related problems (e.g. abscess, dirty hit; past month), and non-fatal heroin overdose (past six months). RESULTS: Data from 6795 first-time study participants were analysed (median age: 33 years, interquartile range [IQR]: 27-40; median duration of injecting: 13 years [IQR: 7-20]). After adjusting for factors including duration of injecting, each five year increase in age was associated with significant reductions in public injecting (adjusted incidence rate ratio [AIRR]: 0.90, 95% confidence interval [CI]: 0.88-0.92), needle sharing (AIRR: 0.84, 95% CI: 0.79-0.89) and injecting-related problems (AIRR: 0.96, 95% CI: 0.95-0.97). Among those who had injected heroin in the six months preceding interview, each five year increase in age was associated with an average 10% reduction in the risk of heroin overdose (AIRR: 0.90, 95% CI: 0.85-0.96). CONCLUSIONS: Older PWID report significantly lower levels of high-risk injecting practices than younger PWID. Although they make up a small proportion of the current PWID population, younger PWID remain an important group for prevention and harm reduction.