Hello, gang!  I just arrived back from Egypt--still jet lagged--but back in the saddle!  You raise an interesting question about the Earth's age.  It's one that I don't think we can answer from the Bible.  Here's my reasoning:

1)  We don't have firm, historically solid dates in the Bible until the time of the kings.  Most everything before 1,000 B.C. or so is rather fluid.  Even the Exodus, for example, is tough to pin down.  1 Kings 6: 1 tells us that Solomon began building the temple in the 4th year of his reign, "In the four hundred and eightieth year after the Israelites had come out of Egypt."  Solomon's reign begins about 970 B.C..  The fourth year of his reign is thus 966 B.C.  Count backward 480 years, and that places the exodus in 1446 B.C., during the 18th Egyptian dynasty.  Using that number as an anchor, we can estimate the dates of other events in Scripture, both before and after the fixed date.  Archaeological evidence suggests, however, that Joshua's conquest was in the late 1200s B.C., placing the exodus 200-300 years later than noted in Kings.  For our purposes in class, we use the 1446 exodus date to be internally consistent with the biblical text, bearing in mind that other significant evidence suggests a later date.  Personally, I'm not bothered by any of this:  it's the normal give and take of research and scholarship.  Ultimately consensus emerges, and even it is subject to ongoing investigation, research and revision.

2) If we have such difficulty with dating obvious historical events, it's even more difficult when one addresses events from "prehistory," Genesis 1-11.  In fact, no one was there to witness the events; geological time has wide margins of error; and you can't take a fixed historical event in the Bible, such as Solomon's reign, and begin counting backward through family lines to arrive at prehistory dates.  Doing so results in creation being on Sunday, October 23, 4004 B.C. (as calculated by Archbishop James Ussher (1581-1656)), a date that appears in many older Bibles, or in 5770 B.C. as calculated by  traditional Jewish scholars.  Genealogies cannot be used for such calculations, for although genealogies are linear, they are not necessarily complete:  witness Jesus' genealogy in Matthew 1: 1-17, which consists of three sets of fourteen generations, from Abraham to David, David to the Babylonian Captivity, and the Babylonian Captivity to Jesus.  In fact, several kings are missing in the second set, David to the Babylonian Captivity (we have the missing king's stories in both Kings and Chronicles).  The three sets of fourteen generations is a narrative device in Matthew, not a comprehensive genealogy. 

3) I would argue that the Bible's "prehistory chapters" (Genesis 1-11) make no attempt whatever at establishing the earth's age:  the chapters are a poetic rendering of creation, not a geological, scientific or journalistic rendering.  Mistaking one for the other demonstrates a fundamental misunderstanding of the text.  

As I teach Genesis in class--and the rest of the Bible for that matter--I'm insistent upon staying within the narrative parameters given us by the text itself.  For example, was the earth created in a literal 7-day period?  The Hebrew "yom"--day--can be a literal 24-hour day, or it can be a "period of time" (as in the expression  "to live in an interesting day").  That given, I'm content to say that WITHIN THE WORLD OF THE NARRATIVE creation unfolds over a 7-day period; outside the world of the narrative is a different story entirely.  Structuring creation in such a way is a narrative device:  it is not meant to be taken literally.  Such narrative devices are common in mythopoeic literature.

God gives us his word in a variety of literary genres.  He does not give it in a series of scientific papers, mathematical equations, or geological insights.  Hence, our obligation as readers is to engage the text of Scripture as God presents it:  as a literary text, if we are to become "educated readers of Scripture."  Personally, I think using the Bible to address scientific questions is well-meaning, but misguided.  Inevitably, we end up looking foolish.