"Does the new decade start at the beginning or at the end of 2010?"
"When does the new decade start?"
"When does *this* decade end?"
I've seen the question posed a few different ways, and I've seen it given two different answers.
The first answer: "the decade begins at beginning of 2010" – the reasoning being that the "Twenty-Tens" or "Tens" decade needs to include to year 2010. This makes sense.
The second answer: "because there was no 'year 0' the next decade doesn't start until the end of the year 2010" – the reasoning being… wait… what?! "Year 0" has nothing to do with this! Are there really people that think that 1990 was part of "the 80s"?! This answer makes as much sense as "it doesn't start until the end of 2010… because pizzas are round."
Besides just being a "10 year period", there is another definition of the word "decade" which specifically addresses this question: a decade is "a period of ten years beginning with a year whose last digit is zero: the decade of the 1980s."
So there you have it. According to me, common sense, and the dictionary, the "new decade" began on January 1, 2010.
(Anyone that claims otherwise, and tries to give you "answer 2" above, either has no clue and is trying to look smarter than they are, or were convinced by such a person.)