Selection acts differently on females and males due to differences in potential reproductive rates, driving the evolution of sex differences in traits, including growth and behaviour. Additionally, selection pressures vary during an individual's ontogeny, with growth being crucial in early developmental stages and reproduction during adulthood, leading to age- and sex-specific behavioural strategies. In this study, we investigated a sexually-size dimorphic spider, the raft spider (Dolomedes fimbriatus), where females are substantially larger than males. We repeatedly observed spiders from juvenile to adult stages, examining boldness, voracity towards prey, and probability to attack. Our findings revealed that females exhibited greater boldness, voracity, and probability to back-attack the simulated attacker compared to males. Notably, the observed behaviours changed during ontogeny, with sex differences in the magnitudes and directions of change, indicating distinct life history strategies between sexes. Moreover, we detected positive associations between body mass or age and behavioural traits, supporting a proposed positive feedback loop between assets and behaviour. While mass and age were not significant confounding predictors in the analyses of sex differences in behaviour, some collinearity was present between sex, mass, and age, so that their effects on behavioural differences between sexes cannot be conclusively disentangled. Repeatability of behaviours was low but significant for boldness and probability to attack, with similar estimates between sexes. These results underscore the importance of considering sex-specific life history strategies in behavioural trait studies.