we taught our allergic son how a epi pen works, from about age 4, with this siblings etc.
we didnt expect him to use it on himself, but be familier with how it works , how to use it and what to do.
we practice as a whole family, sometimes with cousins, (mother allergic to wasps) using out of date ones on oranges. we practice common senerios, such as in town with friends, on the beach, and go over body posisitions, calling for help, getting the meds to allergic person etc etc.
every one knows where we keep the epi pens and the spare allergy meds/pens , so that they can fetch them if required.
the more you practice the easier you are with using it for real,(which we have done)