When introducing Alma the Elder, the text just says that he was "a young man" (17:2). He probably had to be at least twenty to be counted as a young man. In Hebrew, the word for young men is bu-khar-im. In order to be old enough to serve military duty, the young men had to be twenty years old, and so the soldiers are called the bu-khar-im, the young men. From the age of twenty to thirty, they would be called young men. At the age of thirty, they could hold public office. Alma, being called a young man, was probably somewhere on the lower end of that, in his early twenties. He was bright enough that he has been made a priest of Noah. Why, though, as a young man had he been put on this body of very elite aristocratic priests? All I can suggest there is that he might have been the son of a prominent priest in the city of Nephi, and therefore had been placed as a young man on the court. His father may have been one of the leaders who came from Zarahemla down to the city of Nephi.
Is his youth significant? On the Sanhedrin in Jerusalem (and this is coming from a later time period in Jewish law), there were 70 people. After the members of the court had heard all the evidence and had discussed the options, they voted in order of age and seniority, starting with the youngest. The Talmud explains why, "So that the young men will not be intimidated by the opinions of the older, more powerful of the court." We do something similar today in our high councils, where the youngest high councilor will speak first followed by the rest, in order.
Alma’s speaking first, then, was at least consistent with that scenario. Maybe he knew that as the youngest man he was going to have to speak and cast his vote first. That may be why he was paying close attention throughout this event, and why it affected him so much.
When Alma spoke out in Abinadi’s favor, Noah told his guards to get rid of him. Alma fled. Presumably Noah then brought in an alternate priest and filled the vacancy so they could continue. It may have taken them a couple of days to do that; perhaps that took place during their three-day recess.
We can learn some lessons here for our own lives in terms of the courage of young people. For example, our own missionaries courageously put themselves in harm’s way. The youth factor, I think, is always an important and interesting one.
John W. Welch, "The Trial of Abinadi," in The Legal Cases in the Book of Mormon (Provo, UT: BYU Press, 2008), 185.