There is a 25% chance that a child will have allergies if one parent, sister or brother has allergies.  The risk becomes much higher if  both parents have allergies.  Allergies can develop at any age but most commonly show up during childhood or early adulthood.  In almost every case if you have allergies you have a relative who does too and not necessarily to the same allergen.  Call your physician if you need urgent care because of allergies.