Is it unusual to develop an allergy to eggs at age 17?

Egg allergy. All food allergies occur in children at a higher frequency than in adults however; food allergy can develop at any age. Food allergy can occur regardless of how many years it has been consumed prior to the first adverse reaction.
Unusual? Yes. Impossible? No. By that age most egg allergy that generally appears in infancy has resolved. Food allergies that appear in late adolescence & adult life are typically triggered by peanuts, tree nuts, seeds and seafood. But in allergy, as in all medicine, we never say never.