Age.  It has been a really long time since the human race came into existance.  It started in Sat Yug.  Continued through Treta Yug and Dwapper Yug.  Now the human race is going through Kal Yug.  Each Yug can last hundreds of thousands of years.  This gives us an idea of the length of time that the human race has been evolving.  There is no exact definition of all of the Ages as far as timing is concerned.  The creation of a Yug (Age) is based on the following qualities of the human race:-

• behavior,
• prevalent thoughts,
• religious beliefs,
• character and
• deeds. 

When there is a major drift in these attributes the Age changes.  So any Age can’t be defined accurately in terms of length of time.  It is believed and is said so in GurBani that there have been four ages so far from the inception of the universe.  But, the existence of Akal Purakh has been beyond these four ages.