Editing Fast of the Firstborns - Taanit Bechorot (section)

== Those Obligated to Fast ==
Since the reason for the custom to fast is because they were saved from the plague of the firstborns, in which all firstborns in Egypt died, including those who were firstborn only to their father or only to their mother, the custom was established for generations that not only a halachic firstborn fasts, but anyone who has any aspect of being a firstborn, even if there was a miscarriage before him, or if he is a firstborn only to his father or only to his mother.

When the eve of Passover falls on Shabbat and the fast is moved up to Thursday, even a boy whose Bar Mitzvah falls only on Shabbat is obligated to fast, despite the fact that on the day of the fast itself (two days earlier) he has not yet turned 13 years old.