That is an interesting gap in ages. Lehi and Sariah had 4 boys and then stopped having children for over 10 years (approximately 15 years if my presumptions are correct). I wonder why they had such a long span between children. Why did they begin having children again once they left Jerusalem? If Sariah had her first 4 sons over the space of 5 years, that would make Laman about 20 years old when the family left Jerusalem. If Sariah was 15-20 years old when she gave birth to Laman, that would make her 35-40 years old when they left Jerusalem. I would guess that she was closer to 15 than to 20 when she started having children (based on what I presume about their culture). So Lehi was probably in his late 30s or early 40s when the family set out for the promised land. The white haired Lehi that we see in the paintings seems older than that. I bet he looked younger than the paintings portray.
Now, the growth of the family after leaving Jerusalem includes at least 4 more children. We know that Jacob and Joseph are born in the wilderness. We also learn in 2 Nephi 5:6 that he had sisters. That's plural. It could have been more than 2, but Nephi had at least 2 sisters. When were they born? Well, Nephi clearly states that the family consisted of Mom, Dad, and 4 boys in 1 Nephi 2:5 (600 B.C.). In 1 Nephi 18:7 we find out that Jacob and Joseph were born in the wilderness (about 590 B.C. when they first get mentioned). In 2 Nephi 3:1 Joseph is called the last-born. Does that mean last born boy? When were Nephi's sisters born. I'm guessing they were born in the wilderness between 600-590 B.C. That would make Sariah in her late 30s and early 40s when she had these kids. Unless "sisters" is a more general term that included Nephi's nieces? Then Sariah would have only had boys. Maybe the sisters were there all along, and Nephi only reported the boys in 1 Nephi 2:5.
I'm pretty sure the sisters weren't there yet, because of Ishmael's family. Lehi's 4 sons are sent back to Jerusalem to get wives. Ishmael's family fits the bill. On the way back into the wilderness we learn from 1 Nephi 7:6 that Ishmael had 2 sons and 5 daughters. So, Ishmael's 2 sons would maybe match up with possible daughters of Lehi and Sariah, but 1 Nephi 7:6 also informs us that those sons already had their own families. It would seem that Nephi had no sisters at that point. The sisters were born later.
We know that Lehi died between 588 and 570 B.C. If he was 40 when they left Jerusalem that would have him dying between the ages of 52 and 70. He tells Joseph "thou art little... remember the words of thy dying father" in 2 Nephi 3:25. I'm thinking Joseph had to be old enough for a conversation like that, yet young enough to be called little. Maybe he was 10? He was born sometime before 590 B.C. and I think it was about 593 B.C. Lehi says in 2 Nephi 3:1 that Joseph was "born in the wilderness of mine afflictions; yea in the days of my greatest sorrow did thy mother bear thee." Going back to 1 Nephi 17:1 we learn that they "did travel and wade through much affliction in the wilderness; and our women did bear children in the wilderness." Then a few verses later in verse 5 they came to the land of Bountiful around 592 B.C. The land of Bountiful surely wasn't where Lehi had the days of his greatest sorrow. So, based on all of these facts, I would guess that Joseph was born around 593 B.C. That was 7 years after leaving Jerusalem. About 10 years later Lehi died, maybe around 60 years old. Sariah would have been somewhere between 42 to 47 years old when Joseph was born. No surprise that he was the last-born. It seems as though the sisters must have been born between Jacob and Joseph during those 7 years.
So that is what I can surmise about Lehi's family, based on the writings of Nephi and Jacob. I know it's not really eternally significant. There are many more doctrinal studies that will strengthen my testimony, but it really gives me more of a feel for what the situation was. These were real people, going through real experiences. I am grateful for the obedience of Lehi's family and their dedication in following the Lord.