Until they start hassling their parents for Robux cards or stealing their parents' debit cards to buy more Robux. You still get stories of kids buying $1000+ worth of Robux on their parents' phones/debit cards. Those games are deliberately designed so they are not as fun to play without spending money. 

50/50. I see both Pokemon kids and Pokemon adults all the time.

Every time I see Pokemon being a thing it's Millennials and Zoomers with disposable income who grew up on the Gameboy/GBA/DS games are now customers for life.

I'd say it's even. You can see both an influx of new younger fans as well as a lot of nostalgia and interest from older fans like myself.

It seems that majority are adults. Which comes as no surprise, since, apparently, only 21% of Nintendo userbase is under 18.

Adults especially if you talk about the cards you have to have a lot of money if you want to be a serious collector.