I plant some carryover seed every year and never had any issues with it.  I keep it in the original paper bag (it needs to breath, so no plastic) and in an old refrigerator.  I think some I planted last year was 4 years old.  I know some was at least two years old, and some one year old.  Just have to keep it cool and dry.  A lot of field corn seed is carryover seed from last year.