If I hear one more person say that if they hear one more person say that this is the start of a new decade they shall scream, I shall scream.
Yes, I understand basic maths, so I understand that because there was no year 0, technically every decade starts when the year ends in a 1. And here’s the thing: I don’t care. The numbering of our years in tens and hundreds and thousands is entirely arbitrary to begin with, and if we invented it, we get to decide how to use it. Convention and instinct tell us that the change from 2009 to 2010 is more exciting and more worthy of recognition than the change from 2010 to 2011, and they are correct. And if we start our decades with a 1 and end them with a 0, does, say, the year 1930 no longer count as part of the 1930s? Because that way lies crazy, missus.
So when the next person tells you smugly that the new decade doesn’t start for another year, please bop them on the nose and tell them I said it was OK. These people have too much time on their hands. Perhaps we should make decades shorter for their benefit.
Glad All Over 
As another tactic, you could point out that our socio-cultural way of viewing decades means that decades rarely, if ever, start at the beginning of a numerical decade, whether that’s in January xxx0 or January xxx1. The 1920s began in November 1918. The 1930s began with the 1929 crash. The 1960s began in 1963 (‘between the end of the Chatterley ban/ And the Beatles’ first LP’). The 1990s started in 1989, and the 2000s began on 9/11.
It might be that the 2010s really started with the collapse of Bear Stearns in 2008? Only time will tell.