There's an extremely well hidden page on Southwest's site where you can check out their senior fares:  http://southwest.com/cgi-bin/requestFares

Choose the "to" and "from" city and click enter. As you'll see, they're seldom the lowest fare offered, but again, they might not have advance purchase requirements and other restrictions, and they're easy to change. You can only book them by phone (800 I FLY SWA).