When population growth increases in an area what is happening to the birth rate and age structure of that population?

The increase in the rate of population growth in a given area may or may not be related to changes in the Birth Rate, since Immigration also can have a large affect on population growth. If the population is growing because of an increase in the birth rate, then the average age of the population is falling, and young people will probably become more numerous than older people. That is not necessarily the case if the population increase results from immigration.