By faith Abraham, even though he was past age — and Sarah herself was barren — was enabled to become a father because he considered him faithful who had made the promise.
Note: Or By faith even Sarah, who was past age, was enabled to bear children because she
And by faith even Sarah, who was past childbearing age, was enabled to bear children because she considered him faithful who had made the promise.
Note: Or By faith Abraham, even though he was too old to have children—and Sarah herself was not able to conceive—was enabled to become a father because he
Note that essentially the TNIV has opted to go with the optional translational note of the NIV, although the wording was updated somewhat. Readers should know that "Abraham" does not actually appear in the text. The question has to do with whether or not Abraham or Sarah is the subject of "δύναμιν εἰς καταβολὴν σπέρματος" (lit. power/ability to build seed ).The word καταβολὴ/katabole, which I translated literally as "build" in the parentheses is a very interesting word. According to the BDAG lexicon, it means "the act of laying someth[ing] down, with implication of providing a base for someth[ing], foundation." It's also used as a technical terms for the sowing of seed and is used in the Scriptures as a term for "begetting."
So the question here, I suppose is whether or not there's a biological conflict with the idea that Sarah could "build seed" seeing that technically "seed " (we'll keep this G-rated) is "built" by the man, and not the woman. In reality, I don't think there's a conflict here. We're not being presented with a lesson in biology, but rather a common biblical idiom is being used to refer to Sarah's conception even though she was well beyond her years. In my opinion, the attempt at harmononization in the NIV is really not necessary. The TNIV is not only simpler, but closer to the Greek text. Both versions offer the other alternative in the footnotes.
For those who are interested, translations that agree with the NIV include the GNT, GWT, NET (note the use of "he"), and the NRSV.
Translations that agree with the TNIV include the ASV, CEV, ESV, HCSB, KJV, The Message, NASB, NLTse (the NLT1 makes an interesting compromise), REB and the Tyndale NT.
For a defense of the interpretation such as that found in the NIV, see William L. Lane's commentary on Hebrews in the Word Biblical Commentary series.