Why Is Nintendo Considered For Children?

[Barozi]

Because Nintendo advertises themselves as such.

WiiU:
http://www.gamesindustry.biz/articles/2013-12-18-nintendo-targeting-wii-u-marketing-to-kids-families

hell they went even farther with the 2DS:
http://www.polygon.com/2013/8/29/4671052/nintendo-2ds-targeting-demographic-of-very-young-kids-says-nintendo

Other factors being that their games are never or just barely going for a realistic artstyle and even some that do (looking at you Zelda) don't have things such as voice acting. I can't take that serious since I feel like playing a LEGO game.
Many of their games either don't have any story at all or just the same old simplistic "your princess is in another castle" plot. Games like Mass Effect, Heavy Rain, Bioshock, Fallout and so on make you think about history, technology, political systems, relationships, philosophy etc..

Obviously nothing of that means that it cannot appeal to other groups as well and I do like Mario, ... Wars and Pokemon, but it's still mostly a company that centers around children.