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The Skeptic's Annotated Bible notes a contradiction:

Acts 7:4 says that Abram didn't leave Haran until after his father died.

Gen.11:26 says that Abram's father was 70 years old when Abram was born, and Abram's father lived to be 205 (11:32). Clearly, then, Abram was at least 135 when he left Haran.

Yet Gen.12:4 says he left Haran when he was only 75.

Most interesting is their note that they have "Christian Responses (none yet)". Allow me to provide one.

SAB correctly notes an issue that bothered me when I first studied the genealogies of Genesis. Yes, a straight-forward newspaper reading of Genesis gives us an issue to deal with.

Before I deal with it, let me say that there is the possibility that Stephen, whose speech is quoted in Acts 7:4, is just flat out wrong in his rendering of Jewish history. Luke, the author of Acts, is recording for posterity a speech given by Stephen, and as a recorder is only responsible for ensuring that the recording is accurate, not that the original material is. A parallel example might be the New York Times reporting that a politician said something that turns out to be incorrect. Does that make the NYT wrong? Not at all. So long as their recording of the statement is accurate, then the NYT is not in error. Such a politician DID say such a thing, and it's the politician, not the NYT, on the hook when that statement turns out incorrect. Same for Acts. There are no guarantees that every statement recorded is accurate. But I do expect that Luke has accurately recorded what happened and what was said. I'll let the others work out that theology.

Now I don't believe that Stephen is wrong; it's just a possibility that must be admitted before we start.

But here's what I think is really going on here:

The SAB misquotes (or at least truncates) Gen 11:26 when they say that "And Terah lived seventy years, and begat Abram." with a period at the end. What it actually says is that "And Terah lived seventy years, and begat Abram, Nahor, and Haran." That might seem a small thing, but I think the existence of the other names in the genealogy is significant because it parallels another passage with a similar problem (Gen 5:32) which I'll examine first and then back therefrom into my solution. Though the vast majority of the genealogies name one son (at age x, y fathered z), these two both allege (on the surface) triplets. We shall see that beneath the surface we have clues that explain what's happening.

Gen 5:31 says that, "And Noah was five hundred years old: and Noah begat Shem, Ham, and Japheth," leading us to think that he had triplets that year. But such is not the case. Ham is noted as the youngest (Gen 9:24) and Japeth as the eldest (Gen 10:21) and that might be fine for triplets, except for this:

"Shem was an hundred years old, and begat Arphaxad two years after the flood" -- Gen 11:10.

If Shem celebrated his centennial 2 years after the flood, then he was 97 when the flood came. Yet Noah is said to have fathered Shem at 500 with the flood at 600 (Gen 7:6) so Shem should have been 100, not 97, when it came. The fact that Shem is not the eldest give us the possibility that the 100 years was the age of the elder brother only, and that the author did not bother to give us as much detail as we'd like.

We have a similar pattern in Abram's case: Abram, Nahor, and and Haran all born in the same year (from a straightforward reading). Yet it's likely that Haran is older than Nahor, since Nahor marries Haran's daughter (Gen 11:29). If, as Josephus alleges, Abram's wife Sarai is also Haran's daughter, then Haran is likely older than Abram as well.

In each case we have these similarities:

• An allegation of triplets.
• The person in the main Jewish line is named first.
• The person named first is not the eldest
  
• The person named first has demonstrable chronology problems if alleged to be the eldest.
  

So the simple solution is this: the author of Genesis, in cases where only one son is part of the story (c.f. Gen 5:4-28 for a whole busload of them) the age "at birth" is the age of the father when that specific son was born and the others are subsumed under a note that he had "other sons and daughters". In the two cases where multiple sons come into the story they are named together, but the age "at birth" is the age of the father at the birth of the eldest of the three.

Applied to Abe, that means Abe probably left Haran at 75, meaning he was born when Terah was 130, the 70 referring to the birth of Abe's brother Haran, Terah's firstborn.

Is it the correct solution? I don't know. Does it fit the data? I think it does. Does it resolve the contradiction? Yes. If it is a correct reading then Genesis is not in error, we have simply failed to understand the method of the author. I think that's very likely, and in a lot more cases than just genealogies. There are many, many passages where we are assuming that a newspaper-like reading explains everything even as it gives us seemingly insurmountable problems.

But those are a speculation for another day.