SELECT MAX(column_name) FROM table_name

It will return the person whose birthdate is maximum.,

Say if i have 

Kumar 2012-10-01
Rahul  1988-03-31
Santhosh    1955-01-01
Raj Null
Anil null


It will return me 2012-10-01..

So ideally it will return the birthdate of the youngest person among those persons whose birthdates are mentioned.