Final answer:
The adolescent population is expected to decrease in the first half of the 21st century due to declining birth rates, smaller family sizes, and an aging population.
Step-by-step explanation:
During the first half of the 21st century, the adolescent population is expected to decrease. The trend of population changes indicates a shift due to various demographic factors like declining birth rates, smaller family sizes, and aging populations. Historically, there have been periods of both population increases and declines in the United States. Factors such as economic insecurities, changing labor patterns, and evolving family structures contribute to these changes. Moreover, demographic data suggests that the population distribution is changing, with an expected increase in median age and a significant growth in the older age brackets. This shift is likely due to improvements in healthcare, which leads to longer life expectancy, as well as wider access to and use of birth control.