My understanding is because the universe is assumed to by isotropic and homogeneous, proper time and time are the same thing in the case of each and any reference frame, on a large enough scale.
However, if were on a comet traveling along at 0.9C and we were asked to calculate the age of the...
It simply comes down to the fact that the cosmological constant (or vacuum energy if you will) is an incredibly weak amount of energy. It's not enough to overcome gravity.
So just like gravity (which is a weak force) doesn't squash you together when it pulls you towards the centre of the Earth...
Also as far as I am aware, there is an upper bound limit on the number of baryonic matter in the universe, which is where dark matter comes in.
So even if we start to discover a lot more matter out there (ie black holes), it still won't effect this upper bound limit, and still won't effect the...