Studies have shown that Epi pens retain most of their efficacy over time. Two and three-year-old Epi pens, for example, retain about 90% of their efficacy. If you need them (i.e. your child is anaphylaxing and you only have expired Epi pens), they are likely to be effective. This is good news for people who have ridiculously high copays for Epi pens—which ought to be illegal!
More details on the study which looked at the efficacy of old Epi pens can be found below: