Find the average age of the students who have allergies with food and animal types.
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")