Several issues, one OS developers will likely use this to fully identify all users. Apple, Google and Microsoft would rather have legal identities tied to all activity, and this is an easy pretext.

Second, there's no certainty about how courts might interpret compliance. If the intent of the law is to positively identify minors, a user editable field may not be interpreted as sufficient to comply. We don't know what the safe level of identification will be outside of trying the law in court. Who wants to be on the bad side of that?