It seems the default assigned birthday for people who haven't put anything in their D.O.B field, is 31st December. Gav is aware of the bug.
If you are seeing a cached version of the site (browser will be incredibly slow when viewing the forums), please refresh your browser's cache, and that will fix the issue: http://www.wikihow.com/Clear-Your-Browser's-Cache
Thank you!
Matriarch