Fed himself was quite bad in 2010 post AO. Djokovic sucked, but he still had the age advantage. He was 23.
And no 30-31 year old has the age advantage over a 24-25 year old. So 2011-2012 Djokovic definitely had the age advantage.
Let's take Djokovic for example. How many years did Federer actually have the age advantage over him? Only 3: 2007-2009. Since around 2010, Djokovic has been the one with the age advantage. That's a decade now.
So I fail to see how that is just opinion. The numbers themselves...