Basic population data is usually represented as a bar graph most often grouped in five year brackets as shown in the diagrams below. Most "wild" populations show a triangular pyramid shape (Fig 1). For the purposes of discussing human populations it is instructive to look at the different shapes.

1) "Wild" population of animals or birds have a broad base---small height similar to (Fig 1). The broad base represents large numbers of young individuals many of whom die before reaching adulthood. The small height represents "no old individuals". In wild populations "old" individuals generally do not survive. Each individual must be capable of catching their own food and any small problem due to aging means they tend to die before reaching old age.

2) "Primitive" human populations (Fig 1). Have a broad base but more height than a "wild" populations. The broad base as in a "wild" population represents a high birthrates and death rates amoung young individuals. The greater height results from the fact that human society supports old people who contribute to the upbringing of children. Within limits "primitive" human societies can support old and infirm individuals. Fig 1 shows the 1955 population pyramid for Eritrea.

3) "Developed" human populations (Fig 2) have a narrower base which in some cases may be smaller than upper part of the pyramid. The pyramid has a very similar width right up to around age 60 when it tapers off with increasing death rate of older people. However people up to 100+ are still present in the population.
Such a structure is more or less unknown in “wild” and “primitive” populations and in these populations would probably lead to extinction.
NOTE. Pyramids with a base smaller than the top represent a falling birth rate.