So "they" say... But isn't time relative depending on the observer and what is being observed? "They" say time is fixed and finite. What about the astronauts who experience a lesser elapsed time than those they left behind on Earth? Two different elapsed ages, in the same elapsed interval that has passed. We're on Earth, distant receding galaxies are estimated to be so many light years away and that is extrapolated into them being a calculated age in terms of elapsed time. Because they are moving away, perhaps their observed elapsed time is not 13.7 billion years, but a much lesser interval, perhaps only millions of years. Or maybe trillions of years. Maybe scientists are getting it all wrong. Perhaps the universe is infinitely old, and the original size of galaxies was enormously larger than speculated. While still adhering to the Big Bang theory, where they became instantly enormous exploding from a single point. We don't know what we don't know, and I'm not losing sleep over it because it really doesn't matter.
“But isn’t time relative depending on the observer and what is being observed?”
That’s a tricky question. Relativity assumes that there is no universal frame of reference, so there isn’t some outside place that we can measure distances (including time) based on and apply it to all the different frames that observers are seeing.
That being said, if you really start critically examining relativity, there is a strong implication in some of the results that there IS a universal frame of reference. So it may well be that there is a universal standard we could measure time based on, independent of any observers, and Einstein just made the wrong assumption.