The Nintendo fanbase is older and the Switch is appealling to adults.
The majority of Nintendo's fans are 18+ now, even something like Pokemon which is supposed to be a "kids IP" ... I would venture a bet that the majority of people buying the franchise these days are 16-30+ years old, not 6-14 years old.
Time is not a static property. A person who was 8 years old and got a DS for Christmas 2008 and became a Nintendo fan is today 20 years old, that's happened over and over and over again that now the adults outnumber the kids.
Older people tend to have their own jobs and their own disposable income, they don't need to wait for mommy/daddy to buy them a game or system for their birthday/Christmas. An older fanbase simply buys more games and has more disposable income.