We're talking the goal. Actually implementing it successfully is another matter entirely.

You need to be thinking beyond just this law. Once you make it mandatory for all sites to use age verification, you then pass other laws to patch the holes in the system. Look at the DMCA. They made it a felony to bypass copyright restriction technology. They could write a law making it a felony to bypass age verification tools, even if you yourself are over 18. Just like it's currently a felony to use technology to bypass copyright protection tools, even for media that you've legally purchased.