Why is it that the later in age this disease manifest itself, the less severe the disease is?

A five-month-old Caucasian female is brought into the clinic as the parent complain that she has been having ongoing foul-smelling , greasy diarrhea. She seems to be small for her age and a bit sickly but, her parent’s state that she has a huge appetite. Upon examination you find that the patient is wheezing and you observe her coughing.
· Write a differential diagnosis of at least three (3) disorders and explain why each might be a possibility and any potential weaknesses of each differential.
· Why is it that the later in age this disease manifest itself, the less severe the disease is?
· What tests would you run to clarify your differential and potentially come to a definitive diagnosis?
· If the same child was African in ancestry would this change your initial differential? Why or why not?