And easy way to demonstrate this is to imagine three points, A, B, and C., in a straight line, equidistant apart. From A to B is 1 billion miles, from B to C is 1 billion miles. Then, from A to C is 2 billion miles. If A is moving away from B at 1000m/s, and B is moving away from C at 1000m/s, then C is moving away from B at 1000m/s,, but moving away from A at 2000m/s. The further away a give point is, the faster it is moving relative to the observer.

At some point distant from the observer, objects are moving at c relative to the observer, and objects more distant are moving at faster than c relative to the observer. Therefore, any object moving at more than c relative to the observer can no longer be seen by the observer because that light can never travel faster than the velocity of that object relative to the observer.

Since the distance between objects in an expanding Universe is proportional to time, due to the speed of light, the further an object is from an observer, the older it is compared to the light that the observer sees from it.