When can an infant be checked for allergies?

Any Age. If an infant is demonstrating signs/symptoms consistent with an allergic reaction, they can be tested at any age.
Varies. Birth with eczema makes an allergist think allergies are present since it is the first sign, typically, for an allergic child. I tend not to perform skin testing before age of 4 since it tortures the child and it rarely changes my treatment plan. I am not going to start allergy shots on a small baby. Blood tests are highly suspect.