Period, epoch, era, age, aeon all denote a portion or division of time; epoch and era can also denote an event regarded as the beginning of a portion or division of time.
Period is the generic term, designating an extent of time of any length for whatever purpose delimited.
Epoch can denote the starting point of a new period, especially as marked by striking or remarkable changes or events or it may apply to such a new period.
Era applies to a period characterized especially by some new order of things.
Age, usually interchangeable with but possibly more specific than era, is frequently used of a period dominated by some central figure or clearly marked feature.
Aeon applies to an immeasurably or indefinitely long period of time.