Why is it significant to think about the age composition of a human population?
I have eliminated your second question, since only one question may be answered. But it's a good question, so be sure to post it!
The age composition of a population gives us important information that we can put to use. For example, if there are many older people and few young people, we need to consider our policies on birth control. If we have many young people and few older people, there are medical implications, perhaps that we are not sufficiently addressing the health of the elderly. There are social, educational, economic, and political implications, too, since a population of mostly younger people means that there is less social guidance, fewer people to teach younger people what they need to know, and less knowledge available to run businesses or government. Additionally, resources such as raw materials and food supplies are affected by the age makeup of a population, with various age groups tending to consume resources in different ways, and if there is a "bulge" of young people, who are, of course, going to reproduce, this is of concern. Trends like this can be spotted in advance, so that policy-makers can reverse harmful trends.