Ok. I’m a novice in this. But isn’t time relative, and also affected by gravity and velocities? When we talk about the age of the universe, from which perspective are we considering it?
Indeed time is relative. But, the observed distribution of matter in the universe turns out to single out one particular frame of reference (= large scale spacetime coordinate system). Namely the one in which the CMB radiation is isotropic (same in all directions, neither red shifted nor blue shifted). It is with respect to this particular reference frame that the “age of the universe” is defined.
From my poor understanding of a Physics degree: the "age" is just counted from when everything existed in the same space, before it's rapid expansion. Although, can you say it expanded? It didn't expand into anything because space does not exist outside of the universe. Or do we say it 'expanded' because the distance between the constituent parts inside of the universe grew? It's not good to think about really.
