using any logic

Although population dynamics can be roughly assessed as exponential growth processes using actual or assumed birth and death rates, this approach is not accurate enough for most purposes. Major errors arise primarily because a homogeneous population is assumed, where the number of births and deaths is proportional to total population. This simplification may be permissible for bacteria, but for human populations, it obviously makes a difference whether the population consists mainly of children, adults of reproductive ages, or old people. Old people contribute no longer, and children are not capable of contributing to reproduction yet. To correctly compute the number of births per year, the number of women of childbearing age and their fertility must be known. Similarly, it can be argued that the number of deaths per year should be computed using age-specific mortalities. A model accounting for the most important age groups is therefore needed if fairly reliable statements are to be made about the future need for: infrastructural facilities (homes, nursery schools, high schools, universities, hospitals, old peoples homes, etc.), private and public services (education, administration, health, etc.), public expenditure (social costs, social security, pensions, etc.), economic development (work force, consumers, etc.), among many other issues. This model doesnt only apply to human population development but also to population dynamics of animals and plants with different development stages (a similar approach can be used for the description of an insect population [egg, caterpillar, pupa, butterfly] or forest development [tree height classes]). To demonstrate the modeling approach, develop a population model with only three age groups: children (0 15 years), parents (age 16 45), and old people (older than 45 years). This division is carried out primarily to allow correct calculation of the numbers of births. If a more exact calculation (say of potential labor supply) is required, and additional age group (46 65 years) might be included.