Because the name "Nintendo" alone already markets to children as do the majority of their games. The other ads are necessary to try to convince the rest of the people that it's not just for kids.

Nintendo always makes a console for children and then tries to convince adults to buy it too, never the other way around because the kids console is their default starting point.

In Japan at least, 35% of the active Switch users in May were under 14. It's pretty much on par with 3DS which had 35% of active users under 14. Compare this with the PS4 which had 7.5% of active users under 14 in May.

Source: https://www.resetera.com/threads/demographics-for-consoles-pc-smartphones-in-japan-may-2018.52871/

That's bullshit. Nintendo never starts making a project with a target audience in mind. They come up with a concept, and sell it to anyone who's interested in it. I seriously doubt that they started prototyping the Switch with the thinking "we're making this for Kids". No, they were making a mainstream gaming console that could be enjoyed by kids, but is generally aimed at all audiences. 

Interesting source, thanks for that. Horrible gender-split for anything not a phone though. Getting female gamers on board might give a big advantage.

Interesting that the OP keeps making comparisons to the GameCube. Apart from the look of the console itself, Nintendo were going in for some more mature content. They got Resident evil exclusivity and as mentioned, Eternal Darkness, Metroid Prime and Geist. I actually thought GameCube was one of the better Nintendo consoles for third party support, with plenty of mature titles such as Killer 7 and True crime streets of LA to name a couple. I think the Wii would have been a better example of what people might perceive as a kiddy console. Back on topic, Nintendo have done a good job with their marketing so far.

That's actually pretty low to be honest. Only 35% of the 3DS base in Japan is under 14?

That means 65% is over 14, and Japan tends to be a market that skews a bit younger at least for Nintendo it seems. 

The split for 3DS is probably more like 70/30 or 75/25 in the West. 

In the US you can see the Switch split from Nintendo here

15 and under portion of the Switch buyer base is only a mere 10%. Now that number will likely grow as time goes on, but I don't think it will be as much as people think because it's going to be offset by a flood of 18+ year olds buying a Switch as the years go on from the Wii/DS era (kids who fell in love with Nintendo from the years 2004-2009 primarily). That is likely to be a bumper crop of 18+ year olds for Nintendo that will counter balance the under-15 crowd of Switch kids. 

Pretty sure Nintendo released a demographic chart and over 30s male was dominating.

Guess we want our youth back haha

Exactly. Some of the fans don't like the stigma because they don't like the optics. People should play what they want without making excuses. Nothing wrong with playing kid games.

It's actually probably kids that want things like Playstation and XBox more. 

Nintendo's audience is majority adult, an increasing majority at that. Judging by Switch sales, no one seems to have a problem buying it, and looking at movies it doesn't look like to me people have problems like children's characters there either.  All the comic book characters are "kids characters", Spider-Man, Batman, Captain America, Iron Man, Hulk, these are all characters made for kids. 

Tetris might seem as a kids game to some due to the cartoon style, no violence or sex; but it's as mature as it gets.