We have gotten corn seed that is 4 and 5 years old and it grows fine. Usually if stored in a dry place the germination will only reduce by a percent or two per year.

I have found that sunflowers will loose gemination much faster. I planted some 4 year old seed and found less than 50% germinated even when planted at the correct deptha and with good moisture. The deer do love sunflowers and will forage on them all summer. I have even seen them take the entire 4" to 6" head in bloom and chew it down. I plant some mixtures of seed the last week of July and the deer feed on it right through freeze up. Sunflowers, Corn, conola, beans, turnips, sugar beets, oats, and some other brasica mixes. I plant it with a very heavy seed rate and it helps to choke out the late germinating weeds.