SELECT avg(Age) FROM STUDENT WHERE Sex  =  "F"
What is the average age of female (sex is F) students?
[Schema (values) (types)]: | voter_2 |  Student : stuid (text) , lname (number) , fname (text) , age (text) , sex (number) , major (text) , advisor (number) , city_code (number) | Voting_record : stuid (text) , registration_date (number) , election_cycle (text) , president_vote (text) , vice_president_vote (number) , secretary_vote (text) , treasurer_vote (number) , class_president_vote (number) , class_senator_vote (text);