Nephi begins his story with the call of his father, Lehi. That is when things changed for his family.
A friend on Facebook asked the question, "How old was Nephi when the Book of Nephi begins?" I am very glad she asked that because it made me think. Of course, there is no exact answer. We just know Nephi was young and large for his age. Someone later posted comparisons to Joseph Smith and hypothesized that Nephi was around 14-15 years old. The mid-teen years are the years that many heroes in the scriptures are called. In studying other great Biblical leaders, the mid-teen range makes the most sense.
What happened at the beginning of The Book of Nephi? Nephi witnessed his father coming home exhausted after his father saw God. His father began to preach repentance to the people of Jerusalem, warning them of the upcoming destruction. His father was receiving death threats and was rejected by the people. Nephi also witnessed his older brothers reject their father. Nephi said his father prophesied and taught many things. We don't know how much time passed from the time when Lehi first received his revelation and when Lehi and his family left. So, Nephi could have been even younger when Lehi first received his revelation. Lehi was probably in his early 30's, the age when a man was able to preach. So at the beginning of the book, Nephi could have been as young as 8. Laman and Lemuel are not mentioned at this point for they were not old enough to "murmur".
Time had to pass long enough that Laman and Lemuel began to doubt. "It's been years father! You keep saying Jerusalem will be destroyed but it hasn't yet!" They had to be in Jerusalem long enough to read some of the words of Jeremiah, whose words were published throughout Jerusalem.
Nephi was with his family in the Arabian desert. They fled all of their possessions, their friends, their extended family. Nephi was alone with his family, uncomfortable and miserable. Seeds of doubt began in his heart. His father taught him to ask God to receive answers. Nephi prayed and received a testimony that what his Father prophesied was true. Nephi was probably around 14 when this happens. Joseph of Egypt was also around that age when he was thrown out by his older brothers. David was around that age when he was picked by Samuel the prophet to be a future king. (Samuel was also very young when he began prophesying) Joseph Smith was 14 when he received his calling.
Some have made the comparison of Nephi and Joseph Smith and I agree that is correct. I also think Nephi was more of a type of Joseph of Egypt to emphasize the fulfillment of Joseph's prophesy concerning his own descendants. Nephi was the youngest brother (or next to oldest) called to prophesy against his older brothers just like Joseph, the father of Manasseh and Ephraim prophesied against his older brothers. Joseph's family fled to Egypt to escape starvation in Israel, Nephi's family fled Jerusalem to escape destruction. Joseph was separated from his brothers and saved them, Lehi, a member of the house of Joseph was also separated from the other tribes on the other side of an ocean and given a new promised land. This promise land later defended and protected Israel in the latter days.
Nephi was given an important choice at that young age. He could have argued with his father like his brothers who used reason and doubt to rebel against their father. Jerusalem was an old city by this time, the city of the great temple and of King David and Solomon. The Northern Kingdom fell, yet Jerusalem stood. Even "prophets" and the great priests of the nation proclaimed the greatness of their city and that it will stand. Perhaps Laman and Lemuel followed these more nationalistic preachers. Nephi decided to go directly to God and for his faith learned of His mysteries. He was humble and desired to learn of God's mysteries. He was of the age to be in charge of his own spiritual growth. The Jews didn't have Bar Mitzvah back in those days but the moment Nephi received his first revelation was sort of his Bar Mitzvah, the night when he became an adult thus making him more mature than his older brothers.