I'm 48 and still racing MX. I quit racing CC back in 1985 because it was to dangerous. I like nice safe MX where the ambulance is always less than 100 yards away! Other than the years I have had to take off to recover from some injuries I have been racing nonstop since 1978.
Here in California there is a lot of racing for older riders. We have the Old Timers International MX series and the Over The Hill Gang clubs series as well. I am a member of both groups. I ride +45 novice in the Gang and +40 novice in the OTMX.
Here is a link to the International OTMX website. There are clubs from Canada to Arizona.
http://russrr.home.comcast.net/~russrr/Here is a link to the SoCal chapter of the Gang. There are five chapters of the gang, four in Ca and one in Reno,NV.
http://www.overthehillgang.org/