A decade is just a period of 10 years. It can start when it makes sense
For example, someone born in 2003 had the first decade of their life from 2003 to their 10th birthday in 2013. They would now be in the second decade from 2013 until their 20th birthday in 2023.
Or you could talk about decades in the Gregorian calendar. The first decade of the modern era runs from 1 AD through 10 AD, the second decade from 11 AD through 20 AD. So we would be living in the 202nd decade running from 2011 to 2020.
However, the most common use of decades, people group the 10 year period with the same 3 starting digits. For example, 1960 to 1969 would be the 1960s, 1970 to 1979 would be the 1970s. We'd be finishing up the 2010s which end this year and next year would start the 2020s.
A decade starts when it makes sense for your context and ends 10 years later.