I was thinking about the age of the universe which is said to be 13.8 billion years approximately. I read that this is derived from two sources , calculating the life of the oldest stars in the observable universe and from extrapolating backwards the distance which is radius from earth to all directions how far we can "see" which is called the observable universe. I understand that due to redshift and other factors we have concluded that the universe expands as the EM radiation "light" coming from the distant galaxies is shifted in frequency or color for visible light which means the universe is expanding. From this we have derived a model of a universe which began at some point and then expanded , but here is my question, I assume noone knows or it is impossible to know the exact point from which space started in terms of where that area/place/point is now in space as of today, so I assume it is impossible to know the exact place in universe for our earth as there are no fixed reference points and we dont even know the size of the universe, but then how can we know the age of the universe if our observable universe is said to be 13.8 years old but if this measurement is based on the observable universe how can we then know if the rate at which it expands is changing and we don't have a fixed point from which to measure, it kinda feels like the balloon analogy where we can't see the borders of it but we as being a tiny fraction if the inside space of that balloon also don't have a fixed center position with respect to the whole balloon so how can we then know the age of the balloon from the point where it was first inflated to the point where we are now if all we see is matter around us for a certain distance in radius to all directions and its simply being pulled away from us due to spacetime expansion. I assume the problem for me here is I fail to see the reference point, do they assume that all this matter we see now expanding away from us was once very close and so they calculate back the distance with respect to time to the point where there was enormous density and call that point as something "soon" after the big bang ? Sorry for a rather confusing question but that is what happened with me while asking it.