If every part of the universe has aged differently owing to time running differently for each part, why do we say the universe is 13.8 billion years old?

Question

For some parts relative to us, only a billion years would have passed, for others maybe 20?

In: Physics

Answer 1

I assume that scientists mean that every single particle of the worlds existence, has been created 13.8 billion years ago in a single event, thus the age of the cosmos is practically all the same for the different objects it consists from.