I know this is very late but I have just watched it again and even though I am optimistic and like to believe that Cobb is awake. Evidence proves otherwise.
At the beginning of the movie, Cobb spins his spinning top in the real world and it topples after about 10-15 seconds but at the end it spins for about 50 seconds. Now i know that he could have spun it at different speeds but the first time it looks like he spun it faster so theoretically it should have lasted longer than the latter.
Lastly, Cobb entered limbo before Saito did, so why is Saito old and Cobb isn’t? Could it be that old Saito is just Cobb’s projection. I would still like to remain optimistic and believe he hasn’t aged because he knows that he is dreaming and the ageing part is just if you believe that you are awake in the dream.
Alas, we will never know…
Here’s something else to think about…If shooting yourself whilst heavily sedated sends you to limbo. Where would you go if you shot yourself in limbo? O.o That also relates to the film as Saito lifts the gun in limbo at the end. As Yusuf says, who knows how long you can be stuck down there, so it can’t be that easy to just shoot yourself to get out.