It's tricky because they're often thought to have only children in their customer base. Largely because of a console, the Wii, which was released over a decade ago. 15 years, actually. oof
Nintendo's "Kiddy" reputation has nothing to do with the Wii. It is entirely due to Nintendo Of America's
extremely aggressive censorship during the NES and early SNES era. It was not always followed to the letter, but under the policies of that time, anything that wouldn't give a "G" rating in a film today was forbidden, as well as things that wouldn't be a problem in a "G" movie. Any mention of religion was outlawed, as was the concept of death, blood, and (of course) any mention of sex. Besides the incredible limitations of cartridges, this is a big reason several of Nintendo's bigger third-party licensees jumped ship to Sony when the PS1 was released.
Nintendo's been trying to kill this reputation for decades now - the entire reason
Killer Instinct exists is the backlash over the heavily censored SNES version of Mortal Kombat, they made sure to have properly gory games on the N64, and eventually contracted Capcom for an exclusive Resident Evil license for the GameCube. All to shed their "for kids and kids ONLY" image. It didn't work all that well, even though Nintendo now has some of the
loosest content standards.
The Wii was Nintendo being smart - since the PS3 and Xbox 360 were locked in a death struggle for the high-end console market, trying to break into that would be pointless. A cheaper unit designed to be approachable to casuals as well as the traditional gamer market? That could hit an entirely different segment and not have to compete at all!