If I have understood this correctly then its quite a tricky one but should be possible using the generate statement

I cannot test the below MDX but here is a stab at what you probably want

WITH

MEMBER [Measures].[AgedGroupTotal] AS ([Measures].[Work Item Count])

MEMBER [Measures].[AgedGroup30] AS

Sum(

Generate( [Work Item].[System_CreatedDate].[All].Children

,Filter(DATEDIFF("d", [Work Item].[System_CreatedDate].CurrentMember.Name

, [Date].[Date].CurrentMember.Name) <= 30))

[Measures].[Work Item Count])

SELECT NON EMPTY

{ [Measures].[AgedGroup30]

, [Measures].[AgedGroupTotal] }

ON COLUMNS

, NON EMPTY {

[Date].[Month].[Month].ALLMEMBERS *

[Date].[Week].[Week].ALLMEMBERS *

[Date].[Date].[Date].ALLMEMBERS }

ON ROWS

FROM ( SELECT ( [Date].[Date].&[2013-01-01T00:00:00] : [Date].[Date].&[2013-03-31T00:00:00] )

ON COLUMNS

FROM ( SELECT ( { [Work Item].[System_State].&[Active] } )

ON COLUMNS FROM (

SELECT ( { [Work Item].[System_WorkItemType].&[Bug] } )

ON COLUMNS FROM [Work Item])))

Mack