The problem with that is that minors will be looking at porn. You're not going to stop a horny 16 year old.
These laws will certainly take care of age gating the decent, well-willing websites. The shady, probably unmonitored, or even niche websites outside of the mainstream, won't.
Aside from the obvious VPN workaround, this will only push curious teenagers towards worse websites.
As for privacy risks, privacy-friendly methods do exist. Yivi (previously dubbed IRMA) is a proof-of-concept that has been running for a while for privacy-friendly verification and data exchange using signed attributes (https://irma.app/docs/what-is-irma/). A signed "user-is-over-18" attribute is all a website would need to verify a user's age, without any other details.
Privacy-friendly age verification won't stop the other risks and problems these law may impose, though.