Nintendo's kiddie image was just their competitors trying to make Nintendo's greatest strength into a weakness. Nintendo typically excels at making games for everyone. That's why Marvel movies are sooooo popular. Do only kids watch that? No. Everyone does. It takes a lot of skill to make a movie that appeals to kids and adults. This used to be the norm back in the day but now people see it as the exception. This is mostly because Nintendo is the only console manufacturer still from the classic gaming age. (Atari, Hudson and Sega are all gone).
Even in the 80's this wasn't even true. More than 40% of NES gamers were over the age of 18. The kiddie image was always a marketing strategy. It worked for a time. I wouldn't be surprised if it becomes completely discredited by the end of the 9th generation though. I expect Nintendo knows that their competitors are trying to pigeonhole them but you can see Nintendo is fighting it. Why else would they be so adamant about bringing mature games on their system?








