Well, the average age of gamers is over 30 at this point in time. I'm 31, and a game developer, but honestly, I'm finding games less and less satisfying as they continue to cater to the teenage boy in me, who is less and less as each year passes. This year I have played almost nothing, to be honest.
I suspect there would be a large market for some truly mature games (and now I'm not talking about AO so much as games that have mature content, not tits and blood), but it's hard to say for certain because the big companies have decided that all the money is in the 13-16 year old range, and that's where games stay firmly entrenched.
It's true that there's a lot of money there, but I think a lot of it is simply that that is where all the games are being directed. It's a feedback loop, where more money is put in to 13-16, and so more money is made there.
Once developers start leaving childish stories and content behind, and moving on to meaningful and emotional content, or more serious and complex content, we should see a shift towards selling significantly more at higher age brackets.
I was personally disappointed in Heavy Rain because it was the potential for a mature game sullied by horrible controls and interface. Had they simply stuck to more traditional control schemes I have no doubt that the game would have done much better. Of course, that wouldn't have made the writing any better, but at least it would have been a step in that direction.
Honestly, I'm looking forward to indie game development becoming more and more prevalent, where developers not beholden to marketing can influence bigger development by branching out and exploring new territory.
I guess maybe it's not the specific answer you were hoping for, and somewhat meandering, but I do believe it's simply a case of money-in-money-out. If people looked for it, it would be there.