EpiPen is used to treat life threatening allergic reactions only (termed Anaphylaxis). This could be caused by insect stings, food, medications or even contact with latex. I guess you could argue that everyone would be better off carrying one around but usually it's use is reserved to people with a history of severe reaction. If you are to use it properly you would need to be familiar with the signs of anaphylaxis such as a rash covering the body and swelling of the lips and mouth followed by difficulty breathing. Beestings will only cause this reaction very rarely, but if it occurs it would be considered life threatening. Here's an excerpt from EpiPen's website.
What Is a Life-Threatening Allergic Reaction (Anaphylaxis)?
Anaphylaxis is a life-threatening allergic reaction that can occur quickly (as fast as within a couple of minutes). Symptoms of anaphylaxis vary, but can include hives, itching, flushing, and swelling of the lips, tongue, and roof of the mouth. The airway is often affected, resulting in tightness of the throat, chest tightness and difficulty breathing. These reactions can also be accompanied by chest pain, low blood pressure, dizziness and headaches.
EpiPens are for people over 66 lbs. and EpiPen Jr, is for people 33 to 66 Lbs.