Background: How gay and bisexual men (GBM) establish partnership agreements may be affected by several factors, including age. The ability to communicate with a partner about sexual agreements has important sexual health implications for GBM. Objective: To assess differences in partnership agreements among GBM. Methods: We surveyed GBM about their partnerships using a national, anonymous online survey in 2013–14. We compared men who had monogamous partnerships with men who had non-monogamous partnerships, according to age and other factors. Results: Regarding the nature of their partnership with their primary regular partner (PRP), younger men were less likely to have an agreement of any sort and were less likely to have discussed it. Younger men were more likely to report having a monogamous partnership, but they were also less likely to report condomless anal intercourse with their PRP. In multivariate analysis of partnership arrangements, having a non-monogamous partnership with their PRP was associated with being older (adjusted odds ratio = 1.03; 95% confidence interval = 1.02–1.04; P < 0.001). Nearly two-thirds (62.9%) of men with monogamous partnerships had a clear spoken agreement with their PRP about whether they could have sex with other men, largely regardless of age. Although slightly fewer than half the men with self-described open partnerships (46.0%) actually described it as a ‘relationship’, younger men were particularly less likely to do so. Conclusions: Due to less communication with partners about sexual agreements, when young GBM engage in sexual risk behaviour they may be at an increased risk of HIV and other sexually transmissible infections.