alt Hacker Newsnot to be flippant, i am answering your question with the seriousness it deserves:
it is because any government regulation over user identifiers in an operating system (and left to grow and fester according to political wont) will chill free speech (code, data) and assembly (the ability to share code and data with others unsupervised).
That's nice but doesn't actually answer the question that I asked, which is how this (i.e. requiring local user accounts to specify an age range on creation) is bad for a free society.
Simply stating something you apparently see as self-evidently true in the abstract doesn't really make much of a point. Especially when said something is unironically just "but the slippery slope!"