Shudō, or "The Way of the Boy," is the Japanese term for the art of man-on-boy love.
Back during the Edo Period of Japanese History, it became a huge fad to attach the Chinese character for "road" or "way" to all sorts of activities to make them sound more impressive or refined. This character (道) is the same character used for dao in "Daoism." Thus swordfighting became kendō ("The Way of the Sword"), calligraphy became shodō ("The Way of the Pen"), flower arranging became kadō ("The Way of the Blossom"), and beating people up became bushidō ("The Way of the Warrior").
In those days, the practice of adult men keeping young boys as lovers, while not exactly the norm, was fairly widespread, particularly among the aristocratic and samurai classes. Not to be left out of the new fad for calling one's hobby a "Way," participants in this practice took to calling their love of boys shudō or "The Way of the Boy". Drawing upon the millenia of accrued cultural connotations, the use of this character provided an additional veneer of respectability to the practice of man-boy love by suggesting that it was some sort of traditional "Art," a structured body of practice, and a path to inner knowledge and self-fulfillment.
Once man-boy love had been declared a "Way," it became acceptable topic for academic discourses on the history and aesthetics of this "Way," and the methods and techniques best suited to following this "Way." In practice this meant literally hundreds of manuals on the best way for older men to seduce teenage boys (one historian with a lot of time on his hands, Junichi Iwata, was able to count 457 such manuals published in the 17th and 18th centuries alone!). Attempts were also made to trace the "traditional art" of shudō all the way back to Japan's earliest literature, such as the 8th century Kojiki chronicle.
These manuals and monographs helped popularize shudō, and in the later Edo period it became widespread among the urban middle classes, and many of the more popular works were written by authors from these social strata, such as the Osaka ne'er-do-well Ihara Saikaku. In these treatises, the youths would be celebrated for their great beauty, innocence, and unquestioning obedience to their older lover, while the older man would be praised for his caring condescension, and his gentle yet firm educating of the youth on the ways of the adult world.
In actual shudō practice, the youth, called a wakashu ("young one") would be anywhere from about 14 years old to 23 or 24, whereas the older man, called a nenja ("concerned one" or "caring one"), was typically portrayed as a middle-aged man in his 30s or 40s (the beginnings of male pattern baldness was a typical trope). In the act of sexual intercourse, it was expected that the older man would always be the penetrator, whereas the youth would be the penetrated. Once the youth reached adulthood, it was generally expected that he would give up being penetrated and either take up male-female love, or if he wished to continue down the path of male-male love, that he would become a nenja and find a wakashu of his own to teach the ways of the world to.