“All measurements start from zero”
Indeed they do, vivandorron. But years do not. There was not, as far as I know, a year zero. The first bit of your thesis is quite correct. However, if the baby was born on the 1st January year 1, it will be ten on 1st January Year 11. So its second decade begins in year 11.
The second bit of your thesis falls down because from year one, each new decade began in the year ending in a ‘1’ , not a ‘0’. The year 2000 was not the start of a new decade (counting from year 1) – 2001 was. So 2011 is.
Now that IS child’s play !!