Nintendo's fans are majority adults at this point, that's just what happens when you pile successive generations of fans on top of each other for decade after decade, they will begin to outnumber the "children" because you can only have to so many 6-14 year olds lets say at any given time but the aging side of the fanbase is adding millions of "for life" fans every Nintendo product cycle.
Same thing happened with comic book characters, at one time Spider-Man and Batman and characters like that were for (well) children. But today, the fanbase is overwhelmingly adults. Because you have people who were fans from the 60s, 70s, 80s, 90s, 2000s, etc., that outweighs the kids.
Nintendo made "lifer fans" from the NES/GB, then the SNES, then the N64/GBC Pokemon Boom Era, then GCN + GBA, then DS + Wii era, then 3DS + Wii U and even with Switch, if you got a Switch in say 2017 as a child (say age 12) ... you are today either 18 or close to it. Then throw in declining birth rates in the West and Japan, and welp, you get a situation where the fanbase has become like probably 70%+ adults or at least 70%+ 16 and over. In the next 3-4 years you're going to have even more "Nintendo kids" who got Switch as their first Nintendo console graduating up in age to become 17-23 years old.