Hives are often caused by an allergic reaction — those red, itchy welts may emerge within just a few minutes or within a few hours of coming in contact with a trigger such as pets, pollen, certain foods, latex or medications. They may sometimes also appear for reasons you might not guess, such as from stress, exercise, pressure on the skin, too much sun or cold exposure, a bacterial infection or common cold, or from certain illnesses [source: American Academy of Dermatology].
Anyone at any age can get hives, and luckily most cases don't need treatment — at least aside from a cool oatmeal bath and maybe an over-the-counter antihistamine.
If, however, in addition to hives, you also experience trouble breathing, wheezing, dizziness, or a swollen throat or tongue, you may be having a severe allergic reaction called anaphylaxis. Anaphylaxis is life-threatening; go to the ER.