is there good solution on select case? My select query looks very stupid. :))

Something like this could do it with less repetition (or define the age ranges in a permanent table and skip the with clause):

with age_classes (min_age, max_age, age_interval, code, age) as
     ( select 13, 18,  '13-18',  'teen',                   'Teenager'       from dual union all
       select 19, 25,  '19-25',  'young',                  'Young adult'    from dual union all
       select 26, 35,  '26-35',  'still_young',            'It won''t last' from dual union all
       select 36, 45,  '36-45',  'slightly_less_young',    'Middle aged'    from dual union all
       select 46, 50,  '46-60',  'still_got_it',           'Still got it'   from dual union all    
       select 60, 100, '60-100', 'exceeding_expectations', 'Free bus pass'  from dual
     )
select nvl(ac.code,'other') as code
     , nvl(ac.age,'other')  as age
     , nvl(ac.age_interval,'other') as age_interval
     , count(*) total
from   sec_survey_users su 
       left join sec_users u on su.user_id = u.id 
       left join age_classes ac on floor(months_between(sysdate, u.birth_date)/12) between ac.min_age and ac.max_age
group by
       ac.code
     , ac.age
     , ac.age_interval;

With a permanent table, you could define the age_interval label as a virtual column that concatenated the min/max age columns to avoid some redundancy. The table would also be convenient for dropdown lists in reporting applications etc.

You can rewrite the query by using CTE in order to prevent repeatedly writing FLOOR(MONTHS_BETWEEN(sysdate, u.birth_date) / 12) which and age aliased form of it will be used within the GROUP BY lists such as

WITH u0 AS
 (SELECT FLOOR(MONTHS_BETWEEN(sysdate, birth_date) / 12) AS age, COUNT(id) AS total
    FROM sec_users
   GROUP BY FLOOR(MONTHS_BETWEEN(sysdate, birth_date) / 12)),
u2 AS
 (SELECT (CASE
           WHEN age BETWEEN 13 AND 18 THEN
            'Өсвөр үе'
           WHEN age BETWEEN 19 AND 25 THEN
            'Залуу үе'
           WHEN age BETWEEN 26 AND 35 THEN
            'Идэр үе'
           WHEN age BETWEEN 36 AND 45 THEN
            'Хижээл үе'
           WHEN age BETWEEN 46 AND 60 THEN
            'Өтөл үе'
           WHEN age BETWEEN 61 AND 100 THEN
            'Өндөр үе'
           ELSE
            'other'
         END) age,
         (CASE
           WHEN age BETWEEN 13 AND 18 THEN
            'kid'
           WHEN age BETWEEN 19 AND 25 THEN
            'bigger_kid'
           WHEN age BETWEEN 26 AND 35 THEN
            'adult'
           WHEN age BETWEEN 36 AND 45 THEN
            'bigger_adult'
           WHEN age BETWEEN 46 AND 60 THEN
            'big_adult'
           WHEN age BETWEEN 61 AND 100 THEN
            'caption'
           ELSE
            'other'
         END) code,
         (CASE
           WHEN age BETWEEN 13 AND 18 THEN
            '13-18'
           WHEN age BETWEEN 19 AND 25 THEN
            '19-25'
           WHEN age BETWEEN 26 AND 35 THEN
            '26-35'
           WHEN age BETWEEN 36 AND 45 THEN
            '36-45'
           WHEN age BETWEEN 46 AND 60 THEN
            '46-60'
           WHEN age BETWEEN 61 AND 100 THEN
            '61-100'
           ELSE
            'other'
         END) age_interval,
         SUM(total) AS total
    FROM u
   GROUP BY u.age)
SELECT code,
       MAX(age) AS age_level,
       MAX(age_interval) AS age_interval,
       SUM(total) AS total
  FROM sec_survey_users su
  LEFT JOIN u
    ON su.user_id = u.id
 WHERE su.survey_id = 'D3A21B1C2D8334C9E055824F7FFC5DF4'
 GROUP BY code