In the thick of World War I this notice appeared in the Jewish Chronicle (March 3, 1916) regarding the bar mitzvah of Reuben Ginsberg. Ginsberg was born in Wales, but moved to Montreal with his family. His father had been a soldier before, and joined the Canadian Expeditionary Force. When his father was sent to England, Reuben stowed away. When he was discovered he was allowed to join his father's company and was given a job as trumpeter. Sadly, he was wounded. In the period of recovery he turned bar mitzvah, which is what the article is about. At the end of the article it is stated that he is the grandson of the late Rav Shmuel Salant!
This was no imaginary story. Here is his discharge certificate:
Reuben Ginsberg died in Montreal in 1960:
Here is more information about Ginsberg, and underage Jewish soldiers in the Great War.
Here is a photo of his grandfather, Rabbi Samuel Salant (1816-1909) from the Memorial Book of Rowno (1956):
I included this one because the caption amazingly says that this is Rabbi Dovber, Maggid of Mezeric who died in 1772. This is a photograph. According to an article on rabbinic portraiture by Aviad Hacohen this portrait of R. Salant was reproduced and captioned as the Maggid many times. Go figure.