 For example, developing countries with limited access to healthcare and contraceptives, where populations have a high fertility rate, tend to have a lower proportion of older people.Medical advancements, industrial developments, and better knowledge of sanitation, which started in the 18th century in many developed countries, have caused a decline in mortality rates and an increase in fertility rates, factors which raise the number of older people worldwide.According to the United Nations, many developed countries are in more advanced stages (4 or 5) of the demographic transition model and their number of elderly will remain high compared to less developed countries.