I've arranged things with hotels in several states and in a few countries such that my family members who are or were unaccompanied minors have been allowed to stay by themselves at the hotel. I fax over my authorization and my card details and have/had minor-age relative listed as guests in the reservations. When it comes to such stays outside of the US, I even have the hotels send cars to receive them at the airport. The minor then checks in with parental/guardian notarized letter on arrival at the hotel and goes to their room. I've done this for things like camps and weddings. Some hotels are more cooperative than others. If a hotel isn't cooperative, some of us have ways around that in some cities to have a chosen adult to show up to check in the minor. Of course this all comes down to whether or not the family considers a relevant minor responsible/capable enough under select conditions to do this kind of travel. That said, there are plenty of adults who are less capable to travel on their own than many of my relatives traveling at ages 12-14 years old; and there are many adults who can be more irresponsible at hotels or at airports or while otherwise traveling than some 12-14 year olds. Skip the "know your customer" stuff, for this ought to come down to "know your relatives' competency level and risk orientation".

It will be interesting to see how hotels try to enforce minimum age provisions when people can do self check-in with a major credit card. I've checked in non-present guests using self-service kiosks in dozens of hotel locations in the US and Canada; their authorized credit cards worked just fine for that. Eventually even kiosk check-in may give way to phone check-ins that don't even require going up to a staffed counter or electronic kiosk area.