Answer:
In a technical sense, 2011 does start the new decade. It works this way because in the Julian Calendar, there is no year zero. In our calendar, it goes from year 1 B.C. to year 1 A.D. There is no year in between. Therefore, if you count by tens starting from year one, each new decade ends with the number one. For example, 1+10=11. Clearly if you repeat that over and over, you will end up with 2011 starting the new decade.
Some people will say that decades start with a zero because, for example, the sixties were from 1960 to 1969. It is true that the sixties were from 1960 to 1969, but that doesn't agree with the fact that there was no year zero, so technically that is wrong.
So when someone says "sixties" they are referring to the years 1960 through 1969. But when a decade really starts in the technical sense, the last number will always be one.
2000 was the first year. We ran thru all of 2000. That's year 1. 2001= year 2. 2002= year3 and so on. Therefore 2009 was the 10th year or the end of the decade.