Thirteen Years And A Day
The Mishnah Berurah writes (225:6), “There is a mitzvah for an individual to make a festive meal on the day when his son becomes bar mitzvah like the day that he marries. . . .
Chinuch
Bar Mitzvah Seudah in the Gemara
The mitzvah of a festive meal when a son becomes bar mitzvah, and whether meat may be served at it during the Nine Days.