If you're having to ask whether you should or not, you obviously don't have the mental attitude to cheat. It requires a special kind of mentality to be able to compartmentalise your life in such a way that you don't call your wife by the other woman's name and that you always have a valid excuse as to why the wife has been unable to get hold of you.
As others have said, explore your fetish for older women through porn with your wife or pay an escort in your required age range for an hour or two's fun (escorts have no reason to blab to your wife, you're just another client.) I'd fuck the MILF but then again I've cheated on my wife several times (some one night stands, a couple of long term affairs) and managed to keep everything separate.