Time for booster seats in the mezzanine: A study by the League of American Theatres and Producers finds that Broadway audiences are getting younger. During the 1998-99 season, 9.5 percent of theatergoers were under 18, up from 6.9 percent in 1991. Kid-friendly shows like The Lion King and audience-development programs help explain the increase in pint-sized critics, the association says. Overall, the average Broadway theatergoer is 42 years old, with two out of three being female. A small base of fans of the Great White Way (6 percent) attends 15 or more shows a year and purchases 30 percent of all tickets sold. We bet they know all the words to the songs, too.