【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.