What are their last names?
select lname from student where stuid in ( select stuid from has_allergy where allergy = "Milk" intersect select stuid from has_allergy where allergy = "Cat" )
And what about their age?
select lname, age from student where stuid in ( select stuid from has_allergy where allergy = "Milk" intersect select stuid from has_allergy where allergy = "Cat" )