I assume you're talking about a Wyeast "Smack Pack", if so it will DEFINITELY be fine; I've had some old smack packs that actually took almost 24 hours to get going and inflate the bag before.
If it's a "Propagator" pack, it contains about 1/5th the yeast that you need for a normal 19L batch of beer, though and you need to make a yeast starter before pitching to the batch.
White Lab vials, Dry yeast packets, and Wyeast Activator packs can be directly pitched; Wyeast Propagator packs are made to be used with a starter. I find the White Lab vials a better value for this reason as you get a lot more yeast cells for your money.
Adam