This is a great idea. It very compactly implements a barebones parental control system: a parent (with admin access) can assign an age group to a user account, and apps which care can easily check it.

I think it's exactly how such a system should work: apps, sites, etc should declare an age limit, and the user's OS should decide if it's going to give the user access to them. This approach is opposite to having the user to prove their age (and worse, the legal identity) to the web site, app, etc.