rs2012 how to change the field a report groups on.

Have a simple matrix report that has columns values on age group values <16, 16-26, 26-65 and 65+

these display in incorrect order on report.

so I created another group called age order 

     when trunc((sysdate-cpi.birth_date)/365,0)<16  and cpi.gender_type = 'F'then '1'
               when trunc((sysdate-cpi.birth_date)/365,0)<16  and cpi.gender_type = 'M'then '2'
               when trunc((sysdate-cpi.birth_date)/365,0)>=16 and trunc((sysdate-cpi.birth_date)/365,0)<=26 and cpi.gender_type = 'F'then '3'
               when trunc((sysdate-cpi.birth_date)/365,0)>=16 and trunc((sysdate-cpi.birth_date)/365,0)<=26 and cpi.gender_type = 'M'then '4'
               when trunc((sysdate-cpi.birth_date)/365,0)>26  and trunc((sysdate-cpi.birth_date)/365,0)<65 and cpi.gender_type = 'F'then '5'
               when trunc((sysdate-cpi.birth_date)/365,0)>26  and trunc((sysdate-cpi.birth_date)/365,0)<65 and cpi.gender_type = 'M'then '6'
               when trunc((sysdate-cpi.birth_date)/365,0)>=65 and cpi.gender_type ='F'then '7'
               when trunc((sysdate-cpi.birth_date)/365,0)>=65 and cpi.gender_type ='M'then '8'

When I change the column groups matrix_AGE_GROUP to sort on the AGE_ORDER group I get an error.