I'm 45. So when I sustain brain damage and forget the last 7 years, I'll understand*.
On a more serious note, should Nintendo take on more "mature" content? I'd certainly be curious to see what they'd do with it, but it's not something that Nintendo has to do to survive. As others have noted, they've certainly solidified themselves as the providers of content for all-ages, a la Pixar. They could go after the "mature" market, but I'd worry it come at a cost to their all-age market, either through less development resources or to a hit to their public image. Imagine if Pixar put out an R-rated animated movie. While it would certainly appeal to some, it could also cause irreparable damage to the family-friendly brand that Pixar has. Nintendo is known as a provider of family-friendly content, even with stuff like Eternal Darkness and Disaster. I don't know if they want to really risk that image by taking a more "mature" route.
I'm also not sure how much they want to get into a market where they have little expertise and that is extremely competitive, risky, and requires a huge investment, while they have a much less competitive and solid market where they've already laid out their roots. I know that if I'm looking at the competitive landscape, I'd rather be going up against stuff like de Blob, Super Meat Boy, LBP, Sonic Colors, etc instead of having to go against RDR, Gears of War, Halo, Uncharted, GTA, Call of Duty, etc (yes, I know those are from different genres, but they all generally appeal to the same audience).
*Note: I'm actually 27, but was surprised no one had yet taken this route.
Edit: Actually, you know what I want Nintendo to tackle? Space sims and secret agent first person games, a la Thief and NOLF. No particular reason why, just that I want to see those genres return.