For the 40ish set, there really isn't too much around.  The clubs like RA, Tabu, Seven, etc. will generally be much younger.  Some that I know have had success at a local place called Club Monaco on Flamingo.  The country western bars often times have an older crowd if you are into that kind of music.  Stay away from the so called "swingers" clubs.  They are a serious rip off, and what you will find there as a single are generally sub par.  

You might like to try some of the lounges at the casinos as well particularly if there is a nice group playing what is considered adult music like jazz, or perhaps a string ensemble.  Some of the more upscale restaraunts also have nice lounges where the "older" crowd hangs out.  Consider Spago in the forum shops, Cafe Nicole, Ruths Chris on Flamingo particularly late since they have a jazz lounge open to 3am which can be quite crowded.  Best advice is to pick a copy of Las Vegas weekly. I think they have a website as well to see what is going on while you are here.

best of luck

i like to hang around the casino lounges ....i check them out before I go in to see what the crowd looks like, because most are visible from outside. some of my favs are
Shadow lounge at Ceasars, the Mandalay Bay Coral or Reef lounges, the Barge at Ceasars, and the IP. The scene is always changing......so it's a flexible situation.