strtoupper('“T')he Sons of Mosiah”

These four names are given three other times, always in this order (Alma 23:1, 25:17, 31:6). The first three are also mentioned in Alma 31:32. This consistency of name order suggests that it is probably their birth order, making Ammon probably the oldest. However, Daniel Ludlow notes:

The order of the birth of the four sons of Mosiah is never made clear in the Book of Mormon. The listing in Mosiah 27:34 would indicate that Ammon was the first born followed by Aaron, then Omner, and Himni. Also, the fact that Ammon was the leader on their missionary journey to the Lamanites would seem to indicate that Ammon was the eldest (Alma 17–26). However, when King Mosiah asked his people to select his successor, they first desired that Aaron should be their king and their ruler (Mosiah 29:1–2). In this single instance it appears as though Aaron may have been the eldest son.
Brant Gardner -

Brant Gardner

Second Witness: Analytical & Contextual Commentary on the Book of Mormon, Vol. 3