Here is a thread that has some interesting data.
http://tomatoville.com/showthread.php?t=17420
I treat that seeds that are less than 5 years old about the same as brand new seeds. Seeds 5 to 10 years old, I would sow twice as many as I would for newer seeds. Beyond 10 years, it is not outrageous to get some germination, but results are not reliable.
Jim