Pardon my ignorance but is the name of this page correct or should be named "21th century" like the page 11th century is? Andre G. Dias (talk) 06:26, February 5, 2014 (Brazil)

This is actually correct. In English, the numbers 11, 12, and 13 have an irregular ordinal; that is, their ordinal does not match that of the other numbers ending in 1, 2 and 3. So, you have "first" (1st), "second" (2nd), "third" (3rd); "twenty-first" (21st), "twenty-second" (22nd), "twenty-third" (23rd); but then you have "eleventh" (11th), "twelfth" (12th), "thirteenth" (13th). --  Seth Cooper  owl post! 17:45, February 5, 2014 (UTC)
Thank you very much! Andre G. Dias (talk) 06:38, March 29, 2014 (Brazil)