Parasites and pathogens are ubiquitous in that they affect almost any aspect of life. They influence host populations and entire communities, but can also target specific groups of hosts. A fundamental challenge of infectious disease epidemiology and evolutionary ecology is to identify the factors contributing to the emergence of epidemics and the spread of diseases, and to assess their epidemiological, ecological and evolutionary consequences. An important yet often overlooked factor influencing variation in disease susceptibility and disease spread is the age of the host. Whereas phenomena relating host age to infectious disease biology of humans and other vertebrates are well known, little is known about age effects in invertebrates and they are hardly taken into consideration in studies of host-parasite interactions and evolutionary ecology. This stands in contrast with the role of invertebrates in human well-fare (invertebrates are often vectors for human diseases) as well as their contribution to apiculture and aquaculture, the maintenance of biodiversity and ecosystem functioning. Previously I found that host age at exposure directly affects the interactions between the bacterium Pasteuria ramosa and its water flea host Daphnia magna. Here, I will show that similar age effects are present in single and multi-strain infections by the microsporidian Hamiltosporidium tvaerminnensis, a mixed-mode parasite of D. magna. Using three species of Daphnia (two clones from each host species), and two isolates of the pathogenic yeast Metschnikowia bicuspidata, I will also show the generality and diversity of host age effects. I will further show that clearance of P. ramosa in 30-day-old D. magna can occur at progressed infection phases, a rarely documented phenomenon in invertebrates. Finally, I will show how experimental evolution can be used to test if the parasite can adapt to the host age class it infects predominantly. Ultimately, elucidating the underlying dynamics will improve our understanding of disease ecology and virulence evolution.