The only airline I know of that routinely has senior fares is Southwest. On Southwest, senior fares are fully refundable, and they are less than regular fares. However, senior fares are definitely not as low as "Wanna Get A Way" fares.
I'll be interested in what other people post about other airlines and senior fares. I'm assuming that if you do trial bookings on each airline's web site, you will see whether there is a lower fare for seniors.