I think most of your problems stems from using the colloquial layman's analogy to the concept as the scientific concept, itself.
But: spacetime is 13.7 billion years old.
But spacetime is 'all events in the past or future'.
So, how can 'spacetime' itself has a past or a future?
Because its finite, yet unbounded. Like the
surface of a sphere.
What you're asking is analogous to this:
Earth's northern hemisphere has a length of 10000 km.
But the Earth contains "all lines of latitude"
So, how can 'Earth' itself have a north or a south?
Don't make much sense does it?
Earlier you wrote:
Have you ever considered this:
scientists say: 'the age of spacetime is 13.7 biljoen years old."
This sentence makes no sense at all.
How can 'spacetime' has an age? It's timeless. It's time itself. How can time has a clock?
The surface of the earth begins at the north pole, and it is ~40 degrees north of me.
I can have a distance from the beginning, even though the Earth, itself, contains all the lines of latitude. Do you see?