A Boston University graduate student was “a kid who did what kids do” when he swapped songs through file-sharing networks like Kazaa, his lawyer said Tuesday as his copyright-infringement trial began. In only the second music-downloading case against an individual to go to trial, the major recording labels accuse Joel Tenenbaum, 25, of Providence, Rhode Island, of downloading and distributing songs from bands such as Green Day and Aerosmith. The case centers on 30 shared songs, though the recording companies say he distributed many more than that. Last month, a federal jury ruled a Minnesota woman must pay $1.92 million for copyright infringement. Charles Nesson, the Harvard Law School professor representing Tenenbaum, said his client, a graduate student in physics, started downloading music as a teenager, taking advantage of file-sharing networks that make it possible for computer users to share digital files with a network of strangers. “He was a kid who did what kids do and loved technology and loved music,” Nesson said in opening statements. Nesson said the recording companies enjoyed decades of success but were slow to adapt to the advancements of the Internet. “The Internet was not Joel's fault,” Nesson said. “The Internet sweeps in like the way the automobile swept into the buggy industry.” But Tim Reynolds, one of the lawyers representing the recording industry, said song-swappers like Tenenbaum take a significant toll on the recording industry's revenues and on backup singers, sound engineers and other people who make a living in music.