when you force someone to signal status as a minor, you are forcing them to wear a target, hostiles will not have so much work to find minors, now they only have to contact, groom, and offend.
The fact that bill breaks kids down by specific age groups makes it seem even creepier. Want to target 13-16 year olds? Prefer kids under the age of 13? California is helping predators by making sure they can tell which group every child's username falls under!