Zachary Bowman, Sam Jacobi, Drake Jellison, Brooke Sawyer and Tyler Sharpe are among a trend- setting group these days. Each of these Indianapolis youths is an only child, and families such as theirs are the fastest-growing family group in the country. According to the U.S. Census, the percentage of women having only one child rose from about 10 percent to 23 percent from 1980 to 2000.