Yeah with a "default yes" approach it'll be easily bypassed. With a "default no" it would work, ie you simply don't get an account at all until you prove your age.
I disagree. Defaults matter. “It’s illegal” is a firm line parents can hold. And, you don’t need to deter 100% - just taking a chunk out will reverse network effects and cause population collapse.
Plus, expect more services to verify IDs in an effort to combat AI spam.