1. Juveniles of diverse host taxa, including vertebrates, invertebrates and plants, are more susceptible to parasitic infections than adults, a phenomenon still poorly understood.

2. To shed light on the mechanisms underlying this host age effect, we investigated its expression during different steps of the infection process of a bacterial parasite of the planktonic crustacean Daphnia. To do so, we infected juvenile and adult Daphnia magna with the bacterium Pasteuria ramosa and monitored parasite development in experiments specific to different steps of the infection process.

3. We did not find an effect of host age on parasite attachment, but found a strong increase in the penetration time of the parasite into hosts with increasing age. Host age at exposure also affected within-host parasite development, which was delayed in old adult D. magna in comparison to young adults and juveniles. Furthermore, parasite clearance was observed only in old adults, suggesting that old hosts have a better immune response than young adults and juveniles.

4. Our results reveal a step-specific effect of host age on disease progression, giving support for different hypotheses explaining high juvenile disease susceptibility. While these mechanisms are likely to work independently from each other, earlier steps of the infection process influence later steps, and thus may respond more readily to selection than later steps.

5. Our results also highlight that clearance can occur both during early parasite establishment and surprisingly at progressed infection phases, an undocumented phenomenon in invertebrates, where late infections are often considered chronic, lasting until host death.