I've heard somewhere that there is a source explaning why do we say "bar mitzvah" instead of "ben mitzvah". I assume that this source it's not dealing with common knowledge on this (such as the development of this term, etc), but rather conceptually. Anybody knows the source and/or the explanation to this?
R. Yitshak Osterlits writes (1) that this is because בר also means 'outside' in Aramaic. The one becoming Bar Mitsvah, is still an outsider to the realm of mitsvot, since he is only now entering it.
A similar explanation is cited by R. Sh'muel Friedman. (2)
(1) Z'khor L'Avraham (Holon) (2002-2003) p. 118.
(2) Mevaqshei Torah (45) (2007) p. 46.