Talk:Guidebook/@comment-46264514-20200704195050/@comment-25599044-20200705074437

If the birthdate is below the age of 20, you can only make a limited purchase of gems.

Aside from that, the date is needed when submitting the Official Twisted-Wonderland Inquiries Form if you lost contact with your account. (The form requires birthdate info if entered in-game as additional info to prove your account belonging to you!)