Contact Age Formula Field (Salesforce)

/* If no Birthdate is known,
   leave the age blank */

IF( ISBLANK( Birthdate ) , NULL ,

  /* Max age determined by the difference in the
     year number */

  YEAR( TODAY() ) - YEAR( Birthdate )

    /* Make an adjustment based on whether the
       birth month and day has come and passed
       in the current year */

    + IF( (

      /* Assuming there are 31 days in every
         month allows us to make a quick
         arithmetic comparison between two
         month-day-of-month combinations */

      MONTH( TODAY() ) * 31 + DAY( TODAY() ) )
        < ( MONTH( Birthdate ) * 31 + DAY( Birthdate )

    /* Make an adjustment to reach the real age
       by subtracting one from the max age when
       the person's birthday has not yet arrived
       in the current year */

    ) , -1 , 0 )
)