"mirror" universe (you can't say mirror because we are as mirror as it is, we equally mirror universes) is not literally going back. It's impossible to be old from nowhere and then become child. That's BS.
They have same time perception as we, but they go other direction.
I guess, it's like one guy walk right, other go left. Both go forward, both have different "forward", but guy who go left does not go to past, he's not being younger LOL