Don't believe anything. Regard things on a scale of probabilities. The things that seem most absurd, put under 'Low Probability', and the things that seem most plausible, you put under 'High Probability'. Never believe anything. Once you believe anything, you stop thinking about it. The more things you believe, the less mental activity. If you believe something, and have an opinion on every subject, then your brain activity stops entirely, which is clinically considered a sign of death, nowadays in medical practice. So put things on a scale or probability, and never believe or disbelieve anything entirely.
-Robert A. Wilson
Excellent quote -- I used that once for a high school biology class, where kids were stating things as facts that were hypothesis or conjecture.
Anyway, let's see...
There are rocks dated to about 4 billion, and some maybe a bit more, so the planet has to be somewhere more than that. Given the processes that we think formed it (not the same now that I learned in high school!) it can't be, in astronomical terms, much older than that -- more than ten percent older, but less than twenty.
That yields somewhere between 4.4 billion and 4.8 billion.
WRT the Creationist stuff...
Rubbish!
Just mineralogy and crystallography say it has to be at a bare minimum, two hundred and fifty million years. Sea floor magetometry referenced with sedimentology give a minimum figure of two hundred million years as well. Applying all that to former sea floor now up on land yields a figure upwards of 3.75 billion.
Without radiometrics, I don't recall that there's any way to go beyond those. But any (young-earth) Creationist, given that data, has only two choices: either those give an indication of the minimum age, or God is a Deceiver.
The problem with the young-earthers is that they don't grasp the nature of the Old Testament, even when it's intended as history. Many incidents, indeed most of those in Genesis, are meant as history, but they are connected by a number of literary devices which we know, by comparison with similar literature from the ancient Near East, to be unrelated to the passage of time -- for example, even genealogies are not strictly sequential; it was a common practice to skip over any losers, or even list only outstanding ancestors, with the result that what is set down as three generations may in fact indicate three centuries.
So, when you get down to it, nothing in the Old Testament is "history" as we think of it in terms of literature, until the generation before King David.
But something they really don't grasp is the nature of the letter "wau" or "vav", which as a word comes over to us in translation as "and". Here's the important aspect: unless the literary structure so indicates, it does not indicate immediacy. Now, that shouldn't be unfamiliar; in English we may be listing a series of excellent things on a vacation, connecting them with "and" yet knowing that they're separated by hours or days. So consider Genesis 1: it's a royal chronicle, and in a royal chronicle, neither sequentiality nor immediacy are significant; the significance is in the theme concerning the feats of a monarch. Thus we cannot expect the word "wau/vav" to indicate immediate following of one event on another.
So consider: "In the beginning, God created the heavens and the earth. The earth was formless and chaotic, and darkness covered the face of the deep. And the Spirit of God hovered/meditated over the face of the waters."
The first line is likely the title of the chronicle; we might write it as, "How God Created the Heavens and Earth, in the Beginning". The second line we modern Westerners tend to take as a static, snapshot statement, but in the time and culture, is was a statement of condition. An example of that for us would the the statement "The water's boiling": yes, it indicates that at the instant we looked, the water was boiling, but it does not indicate how long it had been boiling, or how long it would continue to boil -- in other words, in terms of time, it indicates an indefinite period. So what does Genesis relate to us here? That the earth was shapeless and chaotic for a period of time. How long a period? The writer isn't interested in that, because it has no bearing on the theme. What we do know is that sometime later -- after another "wau/vav" -- we find that the earth now has a great expanse of waters, and the Spirit of God is checking it out.
Right there is room for years -- millions, or billions, because time is of no concern in this sort of narrative; what is off concern is the theme, which is that the monarch (God) laid the foundation for his realm (the heavens and the earth).
Now a hard-core creationist might just accept this as possible, but then insist that the ensuing six days are literal... but I won't go into that; suffice it to say that Genesis 1 does not require a young earth.
and that I believe firmly in science and nothing else
Come again?
