When was Lehi born? We know that Lehi left Jerusalem in 600 BC. How old were his children? We do not know for sure, but we know that Nephi was "large in stature" and yet "exceedingly young" when they left Jerusalem (1 Nephi 2:16). Perhaps he was sixteen or seventeen years old when Lehi had his vision. It makes sense that Laman and Lemuel would be twenty-two to twenty-four years old. Sam, then, would be around twenty-years-old. It seems they had sisters, as well. These sisters (along with others) fled with Nephi after he and Laman and Lemuel had a falling out (2 Nephi 5:6). So, Nephi may have been the fifth or even the sixth child.
How old was Lehi when he got married? Typically, ancient Israelite men would not marry until they were about twenty-five. In those days, you had to establish yourself as a young man—usually, you had a military duty when you were about twenty and then you had to establish a living and a household. You did not have to have a house, but the alternative was living with your wife’s family. It might well have taken Lehi a couple of years to start his family. If Lehi married when he was twenty-five and Laman was twenty-two to twenty-four when 1 Nephi begins, then Lehi would have been about fifty years old when they left Jerusalem, meaning he would have been born about 650–655 BC.