Currently age of universe is calculated to be about 13.8 billion years.
But according to Relativity matter/energy bends space-time and causes time to slowdown.
And according to Big Bang Theory, as we go backwards in time and get closer and closer to the beginning of the universe,
matter/energy density of the universe increases exponentially.
That means time must slowdown exponentially also.
But it does not make any difference for the observers inside the Universe.
So 13.8 billion years is still would be correct from our point of view.
But an imaginary observer outside of the universe would measure age of the universe much higher than 13.8 billion years.
If so then can we calculate age of the universe from an outside observer point of view?
I think it should be possible to calculate it using an expansion model of the universe (to get the changes in energy density of the universe since the Big Bang) and equations of Relativity.
(Currently radius of the universe is calculated to be about 46 billion light years.
If I had to guess, maybe that means age of the universe from an outside observer point of view could be 46 billion years.)