selecting rows with a certain age

Hi

I have the following query;

SELECT m.name,m.gender,m.dob, (SELECT FLOOR( DATEDIFF( 'date('Y-m-d')',m.dob ) / 365.25 )) AS age FROM members m
 

which gets

David      Male      1990-08-17       22
John       Male      1978-02-15       35

and that all works ok

what i want to do, is extend the query to grab rows when the member is between a set age

but when i do

WHERE age BETWEEN 30 AND 40
 

i get

Unknown column 'age' in 'where clause'
 

any idea why?

thanks