A decade is a time interval, so by that definition a new decade is being started and ended at every single instant of time.

The important question is not when a decade has started, but rather to know what time period people are talking about when they refer to "the 20s". I think most people would consider 2020 (or 1920) to be a member of that set, but 2030 (and 1930) are not.

Open question: When people refer to "the 1900s", does that include the year 2000?

It reminds the index of the first element in an array in different programming languages. Some use one-based indexing, others use zero. Which causes problems (bugs) when porting the program to a language with different indexing.

Your first birthday is also the day you were born, so your 21st birthday is really the day you turn 20.

If you're born at 11pm then it is your birthnight :)

(In French we don't say birthday but "anniversaire" so the day you are born cannot be counted in. You have only one birth day.)

Your 21st birthday is the day Earth is back in the angular position it was in when the event originally occurred for the twentieth time.