Oh, you still don't believe me? Yes, I realize that the verse doesn't mention how old Perez was, but we can deduce that he was about 7. Still skeptical? I hope so. If I was you, I sure wouldn't accept such a claim without solid proof. Well, here it comes. Make sure you've got your Bible handy.
Joseph was sold by his brothers at the age of 17 (cf. Gen. 37:2ff). At the age of 30, he interprets Pharaoh's dreams (cf. 41:46). At the age of 37, the 7 years of plenty cease (cf. 41:53). At the age of 39, after 2 years of the famine, Joseph reveals his identity to his brothers (cf. 45:6). The year he revealed his identity to his brothers, is also the same year in which Israel and his children moved to Egypt. So, Joseph was 39 at the time of the migration. How old was Jacob? Jacob lived in Egypt 17 years, and he died at the age of 147 (cf. 47:28). Thus, since 147 - 17 = 130, Jacob was 130 years old when he moved to Egypt (cf. 47:9). Because Jacob was 130 and Joseph was 39 during this same year, it follows that Jacob was 91 when Joseph was born (130 - 39 = 91). From this, we can deduce that Jacob married Leah and Rachel at the age of 84 since his first eleven sons were all born within 7 years of his marriage (91 - 7 = 84). Joseph was the eleventh son, and his birth marked the end of the second seven-year-period that Jacob had worked for Laban (cf. 29:18-30:26). Now, since Jacob didn't marry until he was 84, we can learn that he didn't even leave his father's house until he was 77! He was 77 when he and his mother deceived Isaac. Surprising, isn't it? What's even more surprising is that he doesn't wrestle with the Angel of the Lord until he's nearly 100 years old! We know this to be true because he served Laban for 20 years (cf. 31:41), and he wrestled the Angel after he left Laban. Imagine, a man wrestling at that age and winning (cf. 32:24ff)! But, let's get back to our topic.
Since we know Jacob married at 84, his first four sons were via Leah, and he had 11 sons within 7 years (by 4 different women), it is obvious that the sons came quickly. It is reasonable to assume from the above facts that Jacob was approximately 87 when Judah was born (cf. 29:31ff). Thus, Judah is about 4 years older than Joseph. Now, we learn some sad things about Judah in chapter 38. He married at the age of 21 (immediately after Joseph was sold; 17 + 4 = 21) and had three sons. He would have to be at least 24 when he had his third son Shelah. The first two sons were wicked and the Lord killed them. Shelah grows up (cf. 38:14), and Tamar is not given to him as a wife. Even if we were to assume that Shelah was "grown up" at the age of 12, this makes Judah 36, and here is where the problem arises. Remember that Jacob moves to Egypt when he's 130, and his son Judah is born when he is 87. Thus, Judah is 43 when he moves to Egypt. Of course, Perez is born illegitimately via Tamar after Shelah is "grown up." If Judah was 36 at the time of Perez's birth, that gives Perez 7 years to grow up (43 - 36 = 7) and have two sons of his own before the migration to Egypt (cf. 46:12)!
Well, it seems as if we're in a dilemma. Obviously, it is impossible for a 7-year-old to have children. But, on the other hand, the Scriptures seem to imply that this is the case. Even if some of my estimates are a year or two off, Perez could not have been more than 11 years old, which is still too young to have 2 sons! So, what now? Is there a solution to this chronological problem? Is the Bible incorrect or is my reasoning above flawed?
I believe the argument above is Biblically and logically sound. Thus, there is only one possible conclusion: not all of the 70 people mentioned in Genesis 46 actually migrated to Egypt in the flesh. They all entered in a figurative sense (which will be explained below), but not literally. There is good reason to believe that this is the proper solution to the dilemma, in spite of its incongruity with our common sense.
In addition to the problem of Perez being too young to be a father, we also have some difficulty with Benjamin fathering 8 sons and having 2 grandsons (cf. Num. 26:38-40) by the time of the migration. Benjamin could not have been more than 32 (and perhaps as young as 23). It would be extremely peculiar for him to have this large family at this young age. Likewise there is a difficulty with Reuben. It would be very unusual for Reuben to take four sons into Egypt with him when he only had two sons the year before the migration (cf. 42:37). If we speculate that he had twin sons immediately (or two sons back-to-back within one year), then it could be possible, but once again it seems to be implausible.
This evidence, in conjunction with the impossibility of Perez having two sons at the time, leads us to the conclusion that some of the 70 must have came into Egypt figuratively (i.e., in the loins of their fathers). While this seems unusual to us, God has spoken of other Bible characters in this fashion. Consider Hebrews 7:9,10. Levi was "in the loins of his father," and he was said to pay tithes to Melchizedek. Levi existed figuratively at this time, just like some of the 70 could have only existed in a figurative sense when Israel and his children entered Egypt. The wording of Genesis 46:26 correspondingly permits this interpretation. Also, remember that this was all ancient history when Moses wrote it down hundreds of years later. The independent family leaders would have been very important in Moses' day, not the exact names of those who were alive when the migration to Egypt took place.
It should be noted that the 70 would later become the founders of independent families (cf. Num. 26). This introduces a new aspect to consider. It seems obvious that they are all listed here, not in regards to whether or not they actually existed during the migration, but simply because they would later become the smaller households of the tribes. If we were to assume that all 70 were alive at the time of the migration, this seems to create a problem since none of the 12 sons of Israel were more than 46 years old when they migrated, and it is very likely that they would have more children while in Egypt. If they did, it would be reasonable to assume that some of these other grandsons and great-grandsons of Israel would also become founders of independent families. But, we have no record of this happening in Numbers 26. It seems that only the 70 became the heads of families. Why would only these 70 found independent families if there would be other grandsons and great-grandsons born in Egypt? This seems very strange, and thus supports the conclusion that some of the 70 were actually born in Egypt and did not exist at the time of the migration. There is no other way to explain the fact that in the time of Moses there was not one of the 12 tribes, except the double tribe of Joseph, in which there were families existing, that had descended from either grandsons or great-grandsons of Jacob who are not already mentioned in this list.