MDX calculated member to band a measure
If you would have the age as an attribute in the People dimension in addition to having it as measure, you could define your bands as calculated members of this attribute hierarchy (calculated members do not need to be calculated measures, they can be members of any hierarchy!):

WITH Member [People].[Age].[0-26] AS
            Aggregate([People].[Age].[0] : [People].[Age].[26])
     Member [People].[Age].[26-51] AS
            Aggregate([People].[Age].[26] : [People].[Age].[51])
     Member [People].[Age].[Over 50] AS
            Aggregate([People].[Age].[51] : null)
SELECT 
  NON EMPTY {[Measures].[Count of People]} ON COLUMNS,
  NON EMPTY 
    [People].[Gender].[Gender]
    *
    [People].[Some].[Other Attributes]
    *
    { [People].[Age].[0-26], [People].[Age].[26-51], [People].[Age].[Over 50] }
    ON ROWS
FROM [MyCube]
WHERE ( ... slicer here .... )


Please note that I kept the border ages 26 and 51 in two bands each, as the names of your bands seem to suggest to do this. You can do this in MDX, as each definition of a calculated member is independent of the others, and hence it is not forbidden to have intersections between calculated members. But you probably would opt to adapt the expressions and only include each age in one band.

The way you tried it does not work, as a set in MDX consists of tuples, while you deliver a string from your IIF expression.