This is the first time we have the naming of the sons of Mosiah.
These four names are give three other times in the Book of Mormon, and are repeated in precisely this order (see also Alma 23:1, 25:17, 31:6. There is another mention of the first three in Alma 31:32). This consistency of ordering the names suggests that there was a reason behind the ordering, which would be expected to be birth order. Thus we may speculate that Ammon was the oldest of the four. 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. (See Alma, chapters 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.” (Ludlow, Daniel H. A Companion to Your Study of the Book of Mormon. Deseret Book, 1976, p. 192).