Lawn seed, can it 'go off'?

I bought some lawn seed last year and got a 'buy 1 get 1 free' and planted a patch last year. Just recently I used some of the seed from the opened box to seed a small patch but none of the seed has germinated - I covered it so birds etc. couldn't get at it. I want to see a much larger area now and still have one unopened box, but think that it might be too old and won't germinate, would it be better to buy turf?