In the ancient Japanese art of shudō, or man-on-boy love, the nenja (literally "concerned one" or "caring one"), was the older man who introduced the younger partner, known as the wakashū ("youth") or chigo ("trainee") to the ways of adult love. It was widely understood that when it came to sexual intercourse, the nenja would always be the penetrator and never the penetratee. The typical depiction of a nenja in Japanese art was of a balding, somewhat portly man in his late 30s or early 40s. Make of this what you will.
In general, the nenja-wakashū relationship was expected to be quite loving, and the nenja was expected to train and mentor the youth in a variety of subjects not only relating to physical love. Nenja were also generally expected to be monandrous, and nenja sometimes wrote flowery "fidelity letters" to their wakashu promising to always cherish and support them in language very similar to a modern marriage vow.