[Avodah] Odd bar-mitzvah custom in Selish ca. 1923

[Joel C. Salomon]

This came up a few years ago when I was talking to my grandfather z"l
about his childhood in Selish (AKA Seylesh, now Vynohradiv, Ukraine).
He was an orphan -- his father died in WWI -- and therefore had his
bar-mitzvah at age 12. He apparently was given to believe this was the
rule for yesomim.

I have a *theory* about the possible source for such a "rule", but does
anyone here have actual knowledge of why such practice existed?  (I'm
aware there's no actual halachik basis, of course.)

--Joel