select agegroup,count(*) as  from
(select case when age<18 then '18'
when age>=18 and age<24 then '18-23'
when age>=24 and age<30	then '24-29'
when age>=30 then '30' else null end as agegroup
from (select year(now())-year(birthday) as age from student)t )x
group by agegroup;