[Avodah] chazakah and probability

[Eli Turkel]

We have several times discussed if chazakah depends on actual statistics

The gemara in Nida brings a chazakah of Raba that a boy/girl at the age of 13/12
has a chazakah that they have physical simanim needed to be an adult.
Tosafot brings a proof (see also shev shematza) from the fact that a boy/girl
can make a vow a year before they become an adult even though we don't know
whether they will have hairs in a year from then.

Nevertheless, SA followed by MA and other poskim state that we do not allow
a bar mitzvah boy to be motzi others in a Torah law unless we have
evidence (R Akiva Eger 2 witnesses) that the boy has simanim. Examples
are parshat zachor, kiddush to be motzi women who have not davened maariv,
halitza etc.

The question is why is this chazakah different from other chazakot?
If we assume the reason is that many bar mitzva boys don't have hair why is
that different from tav lemetav etc?

-- 
Eli Turkel