We count specific items 1 through 10. You count 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 apples. You don't count zero apples.

But, decades of time have always been counted 0 - 9. So, the decades of the 20th Century were: 1900 -1909, 10-19, 20-29, 30-39, 40-49, etc. The decades of the 21st Century have been 2000 through 2009, 2010 through 2019, and now we begin the decade of 2020 through 2029.

The centuries begin counting at the 0 number as well. "The Roaring 20s" were the years from 1920 to the stock market crash of 1929. The 20th Century was 1900 through 1999. The 21st century is 2000 through 2099.

As long as humanity has counted decades and centuries, this has been the correct way to do so. No other enumerating conventions apply.