I'm guessing when TNT redid the help page, they forgot about the birthday system for that login page.1. Log out
2. Go to http://www.neopets.com/help.phtml
3. Click on "My Support" and it will ask you to log in.
4. Log in.
This probably should be regarded as a glitch. It'll probably be fixed soon, and who knows if TNT will allow the accounts to be kept. But in case they do, I figured there would probably be people here wanting to know about it, just in case.
I haven't tried, I don't have any old accounts I want, but best of luck to those who want to exploit it!