An age is a very long period of time. Ages are typically homogeneous, with beginnings and endings marked by dramatic changes and events. Each Age is a specific period of time whose basic design is defined by the Pattern of the Age. The pattern of each Age is fixed by the Great Pattern, but the details of a specific Age can vary considerably from the base pattern. The length of time from one variation of an Age to its next variation is long enough to completely erase all memory and records of the previous Age. Into each Age are born individuals who posses the ability to change the Pattern of an Age in either minor or major ways. These people are known as ta'veren, and can have a great influence on events of an Age. Although powerful, ta'veren are still bound to the Pattern of an Age, as well as the Great Pattern. It is theorized that ta'veren are created by the Wheel as a method to assure that the Pattern of an Age is woven properly. 
—Herid Fel on the Wheel of Time
There are some known contradictions or yet unclear parts. See more here.