First you must ask the question, "What is a year?"
A year is one Earth revolution around the Sun. The Earth rotates on its axis a full 360 degrees, 365.25 times during the course of the year.
Now knowing what constitutes a year, we have to consider mans calendar. Currently, we typically use the Gregorian calendar, but there are many others out there which may not put the date at 2011.
We have decided to agree on a start date and to begin counting from there, for social organization, so we all know when things that are planned for the future will happen, and to keep track of our historical events.
If we were to count years at the estimated age of Earth, instead of only 2011 years back, today would be about:
January 24, 4,540,000,000.
But, if you just want to count from the old 2011 year old mark, it's the 21st Century.