From what I understand, people have measured the rate of expansion of the universe by observing the red shift of galaxies moving away. Then, they took the size of the observable universe and used the measured expansion rate to extrapolate backwards to when the size was very tiny. They found that it took 13.7 billion years to get to the current size. (Please correct me if I am wrong about any part of this)
Why is this process not flawed due to the possibility that the entire universe may be larger than the observable universe? So if in a long time, light from a further point than we can see now arrives at Earth, this will mean that we can observe more of the universe and we will extrapolate a greater age.