Referring to centuries by cardinal numbers, we have "the 1900s" (1900-1999), "the 2000s" (2000-2099), etc.
Referring to centuries by ordinal numbers, we have "the 20th century" (1901-2000), "the 21st century" (2001-2100), etc.
Referring to decades by cardinal numbers, we have "the 2010s" (2010-2019), "the 2020s" (2020-2029), etc.
Referring to decades by ordinal numbers, we would theoretically have "the 202nd decade (2011-2020), "the 203rd decade (2021-2030), etc.
Notice that one of these is not like the others. We "would theoretically" refer to decades that way using ordinal numbers -- except that *nobody actually does that*.
Referring to centuries by cardinal numbers, we have "the 1900s" (1900-1999), "the 2000s" (2000-2099), etc.
Referring to centuries by ordinal numbers, we have "the 20th century" (1901-2000), "the 21st century" (2001-2100), etc.
Referring to decades by cardinal numbers, we have "the 2010s" (2010-2019), "the 2020s" (2020-2029), etc.
Referring to decades by ordinal numbers, we would theoretically have "the 202nd decade (2011-2020), "the 203rd decade (2021-2030), etc.
Notice that one of these is not like the others. We "would theoretically" refer to decades that way using ordinal numbers -- except that *nobody actually does that*.
Decades are 0 to 9. Q.E.D.