Different age classes have already been mentioned at the end of the last section and in Figure 3.6. It is clear that any kind of compartment model can be "copied" several times to include multiple members of the same species. Usually, these new members then represent different genders, different age classes, etc. [35]. A multi-individual model spanning a significant part of the generation span of a given species should then also include a mass exchange of pollutant between neighboring age classes (thereby representing an aging process of the individuals) and a mass exchange between parental individuals and newly born. A simple conceptual compartment model incorporating this idea is depicted in Figure 3.7. There, the population under consideration is divided into four different age classes. Age class 1 represent newly born and very young members of the population, age class 2 represents adolescents, while age class 3 is the only age class which is able to procreate. A pool for dead individuals has been added, too.
Figure 3.7: Simple age class model with four different age classes.