Uijin is the first battle that young people in the Japanese samurai hierarchy participated in.
Although the ages for uijin varied from person to person, in many cases the participants were in their early teens around genpuku (coming of age ceremony) and the parents were inclined to send them to a battle that they would doubtlessly win, wishing for their safety. Motonari MORI had his uijin at the age of 20, which is an exceptionally late example.
Examples of Uijin
Motoharu KIKKAWA, aged 13 (1540, Battle of Yoshida Koriyama-jo Castle)
Kenshin UESUGI, aged 13 (1543, Echigo Province Civil War)
Nobunaga ODA, aged 14 (1547, Attack to Ohama)
Ujiyasu HOJO, aged 15 1530, (Battle of Ozawagahara)
Masamune DATE, aged 15 (1581, Battle with Yoshitane SOMA (the 16th family head))
Ieyasu TOKUGAWA, aged 17 (1558, Battle of Terabe-jo Caslte)
Motochika CHOSOGABE, aged 22 (1561, Battle of Nagahama)
Mitsunari ISHIDA, aged 22 (1581, Battle of Awaji Island)
Having developed a new meaning, the word often means facing something that the person has never experienced before.