Last week I tried to watch the "Shut the Fuck Up PSA" from the National Lawyers Guild, and it was age blocked. I did try to log in, but now you have to VERIFY your age too (obviously I clicked off that shit).
But honestly, alternative frontends like Freetube and yt-dlp might not make the cut anymore:
Mpv crashed, and Invidious instances keep saying "video may not be age appropriate"
Edit: My state is soon to perform age verification on social media sites, so this is probably the cause. I'll just have to VPN to other states or countries until VPNs fail/are blocked.
If you want to keep narrowing it down. You should go to the next step .... a brand new clean OS on a dedicated laptop. Install a new Linux OS, new browser, new profile, do absolutely nothing with any of it, set up a VPN, try the site again.
If that fails, then chances are the ID is being done some other way ... do you have control of your router? Are you accessing the internet through someone's connection? maybe your ISP and your general area, regardless of who is using VPN.
The other possibility is ... Are you tethering on a phone? I notice that phone devices seem to play loose with VPN and due to geolocation built into the device tagged to wifi locating services, phones leak location all the time.