Nintendo has made an effort at getting to an older demographic for years now, but nothing ever seems to come of it. Let's take some time to reflect on its efforts in the past and drink some cocoa.
It all started with Killer Instinct. The game was released in arcades and on the Super NES in the mid-90s (95, I believe, but I'm not sure) and it was supposed to prove Nintendo wasn't afraid to make a game for older players. It kind of worked, but but Nintendo didn't release any more exclusive games aimed at adults until the N64 was released about a year later!
With the N64 came KI Gold. By this time, the series' weaknesses had become pretty apparent and a lot of fighting fans were poo-pooing it. The series still has a pretty serious following to this day, but since no more sequels were ever released, it didn't do Nintendo much good!
In the following year Nintendo got Turok and Goldeneye, along with quite a large number of other mature games, though those two were the only exclusives and they were also the ones that sold N64s. It looked like Nintendo was set to impress people with a mature lineup.
Then came another two year break. The Ocarina of Time and Turok 2 helped to fill in the gap, but Zelda is hardly an M rated series and Turok wasn't as well received the second time around.
In 1999 Nintendo and Rare delivered Perfect Dark. The game wasn't as popular as Goldeneye, but it did well.
In 2000 there was Conker's Bad Fur Day. I'd say more than any other game since KI, this one was heralded as proof that Nintendo wasn't just for kids anymore. Ironically it's a pretty immature (but fun) game.
Then Nintendo launched the GameCube with no M rated games at all. Star Wars was the closest thing it had, and it took about six months to release Resident Evil and later Eternal Darkness. Both were great games, but they were a bit late and Nintendo and Capcom didn't seem to put enough marketing behind them.
Nintendo went on to lose Perfect Dark, Killer Instinct and Conker by selling Rare; Turok went multiplatform, as if anybody cared anymore.
Later in the same year (2002) RE0 and Metroid Prime sold quite a few systems, edging out Microsoft in November, but after that we basically sat back and waited...and waited...and waited.
Finally they brought us a remake of Metal Gear Solid courtesy of Konami and Silicon Knights more than a year later. That was eventually followed by Metroid Prime 2 and the no-longer exclusive RE4.
Which brings us to today. Nintendo's one and only exclusive franchise aimed directly at the adult market is Metroid. Hopefully Geist will make a splash, but looking at all the struggles Nintendo went through to wind up with one franchise that it already had nearly 20 years ago, I wouldn't hold my breath. I think the problem is mainly that Nintendo keeps relying on other developers to come up with the games. It doesn't bother me personally, but Nosferat2's wait and see attitude is definitely the smart one for players looking for lots of mature content.
Edit: noticed a mistake in my timeline. There are probably more.