Is it just tradition or is it written into law that the oldest male slice the Challah, at a joyous occasion, such as a Bat Mitzvah.

I have not heard of that specific tradition, but it is certainly not a written law.

Anyone, including the bat mitzvah girl herself may cut the Challah. It is urgent that all who partake of the Challah perform the ritual hand washing and recite the appropriate blessings.

Warmly,
Rabbi Meth

Original post by ATR