Final answer:
The first stage of late adulthood begins in the mid-60s and corresponds to the age range of 60-70 years, known as the young-old subgroup.
Step-by-step explanation:
The age range for the first stage of late adulthood is specified as beginning in the mid-60s and continuing until death. According to the information provided, the United States' older adult population can be divided into subgroups, with the young-old being approximately sixty-five to seventy-four years old. Therefore, the correct answer to the question about the age range for the first stage of late adulthood is B) 60-70 years.