Tampermonkey extension still works for me to unlock age-restricted videos. Is it having issues for you?

If you don't have it, check out these two links:

https://greasyfork.org/en/help/installing-user-scripts

https://greasyfork.org/en/scripts/521089-youtube-age-restriction-bypass-stealth-version

It's got instructions for nearly every browser. Once you have the extension you can install whichever scripts you like. I mostly use it for verification bypass but there's a lot of other cool stuff you can do too.