Use caseto create an age group description:
select *,
case
when datediff(now(), date_of_birth) / 365.25 > 50 then '51 & over'
when datediff(now(), date_of_birth) / 365.25 > 30 then '31 - 50'
when datediff(now(), date_of_birth) / 365.25 > 19 then '20 - 30'
else 'under 20'
end as age_group
from person
Pay attention to an easier way to calculate age.