The human body can develop an allergy at any time. It is most common for children to develop allergies.
However, they can occur in adults at any time.
The science of an allergy is when your immune system mistakenly identifies a substance as harmful. The allergen stimulates the immune cells to release certain chemicals such as histamine, which then lead to allergy symptoms.
There is some speculation by Doctors in the Allergy field that that late on-set allergies cold be linked to stress, infections, illness and lifestyle.