Allergy means "to change" and react to allergens that were previously tolerated. 

Allergy is a reaction of the immune system to a foreign substance which is harmless. The immune system defends against attackers which are recognized as "nonself" or "foreign" and is protective when it responds to "foreign" invaders such as viruses, bacteria, fungi, and parasites. However, when the immune system responds to a harmless foreign invader such as house dust or pollen, it is responding to a false alarm. 

Becoming allergic can occur after a few exposures or after months to years of exposure. The capacity to become allergic is inherited, but family members are not necessarily allergic to the same allergens. 

Once the "change" or allergy develops, it will continue. Allergy can develop at any age.