The main cause to the world’s ageing population is that there are just not enough births. Now, most couples in various parts of the world are having one child only. For example, until they relaxed the rule a few years ago, due to China's one child rule, the population of the country is growing older. By year 2050, the population pyramid of China would be relatively even on the sides, compared to other countries. The number of young in the future cannot replace the number of people who have grown older. Thus in the near future (about 50 years from now), the ratio of old people to young people would be overbalanced, as the young generation now will be old people, while the new generation will be very few in number due to the lower birth rates.
In some countries, this scenario can be seen very well. People are unwilling to have more children, as both the father and the mother are working, and there is no or little time to take care of their child. The high cost of living is also another reason as to why the people are unwilling to have more children. Some couples even decide not to have children.
The unwillingness to have children means that the birthrate drops drastically. So if this scenario continues in the future, we can expect the number of young people to be very small, as compared to the number of old.
Another cause of this phenomenon is the baby boom of the 1960's, causing a certain generation to have more number of birthrates than the other. However, this cause is not as impacting on the world as the baby boom is more or less confined to the United States.
The definition of "baby boomers" by sociologists and the media is people who were born between 1946 and 1964 in the U.S. That would make them between 36 and 54 years of age now. That period was when the U.S. experienced a drastic increase in the number of births. A reason for that boom is that after World War 2, families in the U.S. started to get together again. The population increased by 50%, from 2,858 in 1945 to 3,411 the next year, 4,300 during the peak year and 4,027 in 1964. Boomers today represent about 28% of the U.S. population. This means that in a few decades, around 1/4 of the world population (may be less due to death from old age) would be older than 65 years old.
Perhaps the most reasonable reason for the ageing of population would be the simplest one; we are living longer. This affects the whole world. The life expectancy is getting higher worldwide. People aged 65 and older have will most probably live for another 16 years, out of which around 12 years would be lived in good health. The basic (and obvious) reason for this is advancement. As we advance in technology and the sciences, we invent more "life-saving" pills and more "life-giving" methods. Healthy living allows us to live to ripe old ages.