Find Youngest User In Database?

How do you find the youngest user in a certain table

select a.illness_desc
from uma.pc_illness a, uma.pc_patient b, uma.pc_diagnosis c
where b.patient_no = c.patient_no and c.illness_code = a.illness_code
and b.age = '21';

i currently have this (it also needs to display their illness which i have done) but i can do only if i cheat and see what the youngest user is, so instead of b.age = 21 i need something that will search the database and find that youngest age

we are using SQL+