MySQL if and ifnull use

SELECT age_range,count(*) FROM 
(SELECT IF(IFNULL(age,0)>=25,"25 Years old and over ","25 Under the age of ") as age_range,device_id FROM  user_profile) AS user_profile
GROUP BY age_range

SELECT IF(IFNULL(age,0)>=25,"25 Years old and over ","25 Under the age of ") as age_range,count(*) FROM  user_profile GROUP BY age_range

SELECT device_id,gender,
CASE 
WHEN  age<20 then '20 Under the age of ' 
WHEN age<25 then '20-24 year ' 
WHEN age>=25 then '25 Years old and over ' 
else ' other ' 
END as age_cunt 
FROM user_profile