Why isn't the universe infinitely old? Why didn't the universe do what it is doing an infinite time ago? What acted on the nothing to make it become a something that we call the universe? I dunno, but let's just call it God and have done with it.
The age of the universe is determined by it's observable expansion etc, going back in time it's believe it started at a single point and the big bang caused the expansion we see now. I don't believe they know what happened before the big bang though, what was there etc so I guess the universe existed but not as we observe it now.
but let's just call it God and have done with it.
That just dodges the question, since "God" must have come from somewhere before the universe existed.