i'm now 39 (and 4 days!), and have never had a wet dream. sexy, sexual, horny, explicit, and/or coded dreams, yes. but never wet.
like a few of the guys here, i subscribe to the 'frequent-masturbation' theory as to why not, with a little of the 'big-shooter' school of thought in there as well. i started masturbating when i was around 5, when i discovered that if i rubbed down there for long enough a wave of really nice sensation would come over me. but it was something like 8 years before my hard work started to be rewarded with real cum, so i guess that by the time my balls started manufacturing the stuff there was a ready and well-established delivery channel!

a daily flushing (sometimes twice ... three times ... ) probably meant that the automatic system either didn't need to kick into gear, or didn't have anything to work on.
i've also always considered myself on the generous side when it comes to cumming, too (though advancing years have meant there's less risk to my eyes now :P ), so that may have maintained an empty tank at night.
incidentally, our night-time hard-ons aren't always sex-related: in one of the phases of sleep (is it REM?) our muscles go into paralysis, and one of the effects of this is an erection. i think that can be where the familiar dream of trying to run but having feet of clay comes from: you're trying to move your muscles, but you can't, and that gets translated into your dreams.