The App/Play stores already basically have that, you can't submit apps without age ratings.

One spot where it's a bit weird is for utility apps that don't have any "mature" content but that obviously isn't intended for kids, so e.g. your tax prep app gets rated as "Ages 4+".

Is exposure to a tax app going to cause harm to a 5yo? This doesn't seem like a real problem.

Yes, that's why the system should be a bit different.

It should indicate notable features that influence the rating, like PEGI includes "online content" "gambling" and so on