Modification of DAX for Aging Calculation Based on Selected Month

InsightSeeker 

You can't do it as a calculated column because those are only updated when the model is refreshed, not when a user makes a selection on a slicer.  You can also simplyfy your SWITCH a bit by starting from the top.

Aging Measure = 
VAR _EndDate = MAX('Calendar Ultimate'[Date])
VAR _Due_Date = SELECTEDVALUE(Due_Date[Due_Date])
var _datediff = DATEDIFF(_Due_Date, _EndDate,MONTH)
RETURN SWITCH(TRUE(),
    _datediff >12,"Above 12 Month",
    _datediff >= 10, "10-12 Month",
    _datediff >= 7,"7-9 Month",
    _datediff >= 4,"4-6 Month",
    _datediff = 3,"3 Month",
    _datediff = 2,"2 Month",
    _datediff = 1,"1 Month",
    _datediff = 0,"Current",
    _datediff < 0, BLANK(),
    "0")