Final answer:
The fastest growing U.S. population subgroup is those 85 and older, due to the aging of the baby boom generation and increased life expectancy.
Step-by-step explanation:
The fastest growing U.S. population subgroup is those 85 and older (D). This group has been witnessing significant growth, as demonstrated by various population charts and Census data. Factors contributing to this trend include the aging of the baby boom generation and increases in life expectancy. By 2030, all Baby Boomers will be age 65 and older, representing a significant portion of the elderly population. Cultural values and attitudes also shape the experience of aging, which varies from country to country. However, the 'graying of America' refers to the growing percentage of the U.S. population that is entering old age, especially marked by increases in those over 85 years old.