2 Nephi 2: 1

1 And now, Jacob, I speak unto you: Thou art my firstborn in the days of my tribulation in the wilderness. And behold, in thy childhood thou hast suffered afflictions and much sorrow, because of the rudeness of thy brethren.

According to the date in the chapter description, these blessings to Lehi’s sons were given anywhere from 12 to 30 years after they left Jerusalem in 600 B.C. So the oldest Jacob could be would be 30, since he wasn’t born until after they left. And that is assuming Sariah was pregnant when they left. He probably is not as young as 12, since his younger brother, Joseph, was also born in the wilderness (see 1 Nephi 18: 7). We also have mention of sisters, but we don’t know how many (see 2 Nephi 5: 6), nor do we know how old they are. It’s possible that they were married to the sons of Ishmael, but I think that would be extraordinary, since the sons of Ishmael were sided with Laman and Lemuel (see 2 Nephi 4: 13). We hear about the sisters in context of leaving the main body of Lehites, and culturally I think it would be highly unusual for them to leave their husbands to follow Nephi. So I’m going to assume that Lehi had more children born in the wilderness than just Jacob and Joseph, or at least over the course of 30 years since they left Jerusalem, and they were eight years in the wilderness before they got to Bountiful (see 1 Nephi 17: 4). We don’t really know how long they were in the Promised Land before Lehi died, but I’m guessing that these sisters were young enough to not be married. At any rate, with all these clues, I’m guessing that Jacob and Joseph were teenagers or in their early 20s, or at least that’s what I always picture in my mind. He was definitely old enough to have had some significant spiritual experiences. They were still pretty young while on the boat, according to Nephi’s account (see 1 Nephi 18: 19). So I’m guessing Jacob and Joseph were probably teenagers at the time this blessing was given.