What is their average age?
SELECT avg ( T1.age )  FROM student AS T1 JOIN lives_in AS T2 ON T1.stuid   =   T2.stuid JOIN dorm AS T3 ON T3.dormid   =   T2.dormid WHERE T3.student_capacity   =    ( SELECT max ( student_capacity )  FROM dorm ) 
average age refers to AVG(T1.age); their refers to students living in the dorm with the maximum student capacity;