Infinite Altitude:
What I believe does not matter. The universe could be thousands, millions, billions, trillions of years old or infinity years old. Some theories hold that the big bang occurred at the end of the previous universe.
Black-Hole Big-Bangs
When this universe dies, it may end with a universe-wide black-hole which may cause another big bang. On top of that, there may be parallel universes, dimensions, timelines, and realities. Some suggest that each black-hole takes matter into another universe, perhaps even creating new universes.
Worm-Hole Star-Gates Into Universes
A worm-hole may be a bending and a shortcut not through folded paper but through the dimensions of reality of space and time itself.
Is Time Constant?
On top of that, if time is a dimension, then time may not be constant as mentioned by Einstein. How old something is can depend on how time functions.
The Universe is a Crime Scene
Looking back into the gates of time, at the stars, at world history, at dinosaur bones, at all of these things through not observational but historical science, that is like a detective looking at a crime scene.
Being Framed For Murder
The detective may see the gun in the murder case and yet was the suspect framed or did a particular person do the killing with the gun?
Was the Universe Framed?
Was the gun planted in the crime scene as you would see in Dexter Morgan and Ray Donovan? Was the fingerprints placed onto the murder weapon? A security camera would help determine whether the alleged suspect was framed or not.
Universe Security Camera
Can we get a security camera for the universe or can we trust the alleged suspects of the universe? Are we sure that the universe was not framed?
So, probably somewhat more than 5000 years old.
They say there are these old tablets, like stones, with what may be a list of reigns of these ancient kings or creatures or gods or whatever, each lasting around 30,000 years each or many thousands of years, something like a few dozens of these giant monarchs. Is it possible that these ancient myths, legends, fables, or whatever they may be, could they be completely or partly accurate? Were some of them Egyptian Pharaohs?
Less Than a Billion Maybe
I say it is possible that earth is over ten thousand years old at the youngest. I want to believe that the earth is less than a billion years old and possibly under a million years old. But I don't really know because I was not there. So, I will not say it is a certain age.
Why would you hazard a guess at the age of the earth being "less than a billion" if you believe this question is beyond your epistemological limit?
Stones are notoriously difficult to accurately date, but I find the idea of a "civilization" more than 10,000 years old intriguing.
same thing can be said in reverse
Because of stars.
And the movement of stars.
And the speed of light.
You are ignoring higher dimensions. You are ignoring the possibilities and potential theorized by scientists regarding what these other dimensions may be able to do in regards to folding and bending space, the three main dimensions, and time, the fourth dimension, on top of other possible dimensions. Outside of that, are you certain how big and far away stars may be or are you only guessing.
We only know what we can observe and measure empirically.
Speculation is fun, but we must be careful not to conflate the hypothetical with the actual.