There are roughly four stages of the demographic transition theory. The first stage is before modern industrialization such as the advancement of medicine and the improvement of food distribution. This stage has dominated human history until about the 1800s with high death and birth rates and resulting in a population having a slow population growth. Countries that are before development are included in this category.
The second stage is the start of industrialization in a specific nation. For example, the development of modern medicine and food distribution is starting to take hold and the death rate starts to diminish. The birth rate continues to remain relatively high because the society is still adapting to the rise of new technologies. Developing nations would be included in this category and have a significant population boom because of the lower death rates. These nations also produce a difference in a ratio of age groups because the quality of life is better.
The third stage is the end of the industrialization and the birth rate starts to decline along with the death rate. This is attributed to the society adopting several new ideals such as the rise of birth control, women's rights, couples having fewer children due to economic conditions, and other factors. In a sense, the birth and death rates reach low rates and allow a more stable population. Developed nations, such as the United States at one time, would fit in this category.
The fourth stage is the final stage in the demographic transition theory. At this time, developed countries have birth and death rates that have stabilized and produce a very small rate in population growth. Many developed nations will have societies that have a very high amount of older age groups. The United States today is in this stage with its low death and birth rates and very small population growth.