One of the top priorities for the next president of ABC News won't be figuring out who should eventually succeed Diane Sawyer as anchor of "World News Tonight." It will be deciding whether there will be an audience worth pursuing for "World News Tonight." Not only is the network news audience shrinking, it is also aging. The networks must groom a new audience if they are to survive. This is particularly true for the three evening newscasts, which all start at 6:30 p.m., a time when many Americans are either still working or commuting home. "My students do not watch the evening news," said Christopher Harper, a former foreign correspondent and investigative producer for ABC News who now teaches journalism at Temple. "The evening news has become the province of old guys like me."