Get ready for a new old age. With the U.S. fertility rate in a decadelong slump and the life expectancy of 65-year-old Americans approaching roughly 85, our aging nation is likely to grow older by midcentury, as the ratio of young to old continues to decline. The trend is likely to upend how our society is organized, making life very different for today’s 30-year-olds when they reach their 60s compared with life for 60-year-olds now.