Very early in Nephi’s record, he records that the Lord told him that he would become "a ruler and a teacher" over his elder brothers if they rebelled against him (2:21). One may wonder if this blessing was kept confidential by Nephi, at least as first. While this promise certainly gave Nephi confidence and reassurance, it would not have pleased Laman and Lemuel. Being their "teacher" would be one thing. But being a "ruler" would have been something altogether different.
Examples may be found where younger brothers emerged as chosen spiritual leaders within Israel. Jacob was younger than Esau. Joseph was younger than many of his brothers. Ephraim was younger than Manasseh. David, a youngest brother, became king of all Israel. So, there was precedent for this unusual order of things. Normally the rights of primogeniture gave the firstborn son precedence. But in the law of the Lord, righteousness and calling prevailed over birth order, although understandably not without awkward relations and hard feelings.
In our times, the goodness of Hyrum Smith provides extraordinary exception to the normal problems often generated by this age differential. Hyrum was uncompromisingly loyal to his brother Joseph. Hyrum’s love for Joseph was profound, and his sacrifices were complete. Yet Hyrum was six years older than the Prophet Joseph, his brother and leader. "In life they were not divided, and in death they were not separated!" (D&C 135:3).