I posted this on Youtube's page as well. They recommended removing the channel, rebooting the Roku, and reinstalling.
I tried that, and now it's slightly different.
My normal Youtube account (original name) still doesn't work, though the error message is slightly different.
The account they made us create (with my real name, that I don't actually use) will be able to get through the age-restriction check. I've read others that get this same behavior.