Today president-elect Obama lunched with the three living ex-presidents and one living president [brietbart]. Aaron thought that four seemed small [twittr]; so I looked at how the number of living ex-presidents varied over the history of the republic.
I made this pretty hastily, but even without accounting for the time spans it looks like our current situation is a bit above average. It will be even moreso after the next inauguration.
(data from “List of U.S. Presidents by Age” [wiki])