They do not mandate what x is, but they do mandate that you must say a specific age in any Internet-capable OS, and that age must be communicated to any software asking for it. Either comply with the law and utter an age, or you're breaking the law if you communicate online without such an utterance. It is using compelled speech to threaten your right to all digital speech.

There are cases where compelled speech is considered constitutional, but surely not when it's used to threaten the primary way people communicate.

And contrary to your claim, COPPA does not legally require service providers to ask every user how old they are, nor are users obligated to reply. Age questions are just how service providers chose to implement it. And I don't think COPPA is a good example of a law working as intended.