This model comes with GoldSim software. Instead of downloading the file from here, you can access it by going to the File menu in GoldSim and clicking on "Open Example..." then browse to the model file in General Examples.
It is often necessary to model age-grouped populations of items that move successively from one group to the next as they age(referred to as "aging chains"). For example, you could track the number of individuals in a city by age (1 year olds, 2 year olds, etc.). Each group is referred to as a cohort. During the simulation, each cohort may shrink (e.g., due to deaths or emigration) or grow (e.g., due to births for the first cohort or immigration). At certain periods (e.g., every year), each group "graduates" to the next group (e.g., 1 year olds become 2 year olds).