Question: What is the average of those 3 ages?
SQL: SELECT avg ( age )  FROM Student WHERE StuID IN  (  SELECT T1.StuID FROM Has_allergy AS T1 JOIN Allergy_Type AS T2 ON T1.Allergy   =   T2.Allergy WHERE T2.allergytype   =   "food" INTERSECT SELECT T1.StuID FROM Has_allergy AS T1 JOIN Allergy_Type AS T2 ON T1.Allergy   =   T2.Allergy WHERE T2.allergytype   =   "animal" )