Copyright holders have complete freedom to license tehir work under any terms they like. I believe it's actually legal to sell commercial use licenses for GPL'd code for example. Or to switch licenses for all future releases.

You just cannot revoke existing license terms.

GPLv2 says:

  Activities other than copying, distribution and modification are not
  covered by this License; they are outside its scope.  The act of
  running the Program is not restricted, and the output from the Program
  is covered only if its contents constitute a work based on the
  Program (independent of having been made by running the Program).
  Whether that is true depends on what the Program does.

I can see several possible readings. The first sentence seems to regard "use" as out of scope, and therefore, MuseScore could be well within their rights to restrict it. But the second sentence then seems to grant an unrestricted right to run the program, which this policy denies for people under 13.

edit: GPLv3 says:

    All rights granted under this License are granted for the term of
  copyright on the Program, and are irrevocable provided the stated
  conditions are met.  This License explicitly affirms your unlimited
  permission to run the unmodified Program.  The output from running a
  covered work is covered by this License only if the output, given its
  content, constitutes a covered work.  This License acknowledges your
  rights of fair use or other equivalent, as provided by copyright law.

I can't see this being allowed under GPLv3. And the GPL is pretty clear that in "GPLv2 or later" situations, it's the user who chooses.

The GPL is a contract/license; it cannot trump the law itself. You can’t slap a GPL license on porn and then distribute it to minors.

It may mean you can’t distribute at all if you can’t comply with the law and the license.

good thing there's absolutely no reason to store any data about users of an offline desktop app!