For using crosstab in table pivoting it is recommended that all categories have at least one value, for garanting this we use $$VALUES ()$$ defining the category descripcion

SELECT *
FROM crosstab(
 'select mil_cagr
     , case when extract(year from age(mil_f_nac)) < 26 THEN 1
            WHEN extract(year from age(mil_f_nac)) > 25 AND ...