Each reference might give us less or more. My point is that all of our measurements of time are made from our perspective hear on Earth. We can only say that the universe is 13.7 billion years old as measured by someone on a planet that has the same cumulative velocity of Earth. Someone on another planet with a greater accumulative velocity would presumably view the universe as being much older than we do.
All observers however would agree on the spacetime interval between now and the big bang.
If you draw two points on a sheet of paper, depending on what coordinate systems you use the points will have different numerical values. However all coordinates systems agree on the actual distance. It's the same in relativity, people can use different "time" coordinates, but they'll still get the same spacetime distance.
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