Latex or water-based paint typically has an unopened shelf life of 10 years. Solvent or oil-based paint can last up to 15 years unopened. You can test the paint to see if it's still good by doing the following:
Latex Paint: First, smell it. If it smells rancid, it's gone bad. Second, check to see if the paint is separated. There will usually be a thin skin on top with a layer of liquid underneath. Remove the skin and use a paint stick to stir the paint. Make sure that the paint isn't hard on the bottom or the sides. If the paint blends together smoothly and appears to be the original color, then it can be used. If you're still unsure, brush some of the paint onto a piece of newspaper. If the paint comes out rough and lumpy, you need to dispose of it.
Oil-Based Paint: If the paint hasn't been exposed to extreme temperatures and was sealed properly, then it typically isn't as susceptible as latex is to going bad. Just remove the layer of film across the top and stir it well before painting.