If the seed has been kept in a cool, dry, dark location, it could last for many years.  You can try a germination test by putting some of the seed in a wet paper towel (folded over the seed to ensure the moisture covers the entire seed).  keep them wet for several days.  Most grass seed will take less than 7 days to germ, but older seed may take up to 2 weeks.  You will notice a growth coming from one end of the seed.