Oh come on, you know you need some years to grow and mature before raking up slams, he's playing at his peak now and has won 5 slams in two years. But he's just 25. What makes you think he won't win many more slams in the next 3-4 years?
Plus it's not like the other players in the Top 15 are younger than him, quite the contrary.
I'm not saying he can't do it, I'm just saying that people are being way too casual in saying that he will get to 12 slams. He has to double his slam tally right now at 25, will be 26 when RG comes around. I'd say he has about 1-2 years max to dominate like he did in 2011 and he hardly ever brings that level to court nowadays.
And the 5 slams in 2 years is a bit misleading in my opinion. 3 of those 5 were won in one year on the form of his life, and 3 of those 5 were won on one surface [AO]. Yes he can do it but realistically he won't do it I think, especially if Nadal wins more slams. I'd give Djokovic 10-11 slams max. Winning slams is not an easy feat.