many of you questioned whether Cobb is still dreaming or awake in this film's ending.
the answer is, he is awake and in reality!
-Cobb has an experience living in a limbo with his wife before, and they were confuse which is real and which is reality. But at the end, Cobb realize it, and they both do suicide at the train's track. Since then, they both directly awaken in reality.
conclusion's : no matter how much the layer they're dreaming in limbo, once the died, they will directly awake in reality!
in the ending, Saito died earlier before Cobb go find him.
As Saito died earlier, Saito were living in the limbo earlier than Cobb. Thats why Cobb were younger than Saito when they meet at meeting room which showed twice in the film
(beginning and near ending of the film)
remember, more deeper the dream layer, the time in the dream will be more compounded.
So, eventhough in the 3rd layer dream, Saito died just few minutes before Cobb go find him, but for Saito, the few minutes in 3rd layer dream will give him enough time to grew old in limbo.
Then, after many years living in limbo (which actually just few minutes in the 3rd layer dream), Cobb found him, and then, Saito remember his word with Cobb which is 'Comeback, so we can be young man together again'
and now Saito realize that he is still dreaming.. and look at the gun.(this shows that they both killing themselve in the limbo)
Dead in Limbo brings directly to reality.Thats why they awake in the plane later.
-The other proof is Cobb totem's
in dreams, this totem will spinning perfectly and will never be stop.
in the endings, Cobb spin his totem and left it behind to go for his children, then, the cameras shows that his totems wobble and will stop spinning.This shows that Cobb is in reality, because in dreams, the totem will never wobbled or shows sign that its gonna stop spinning.
-My last point is Cobb's childrens. In Cobb's memories, his childrens never turnback and see his face, but at the endings, his childrens, both turnback and go towards their father (Cobb) which never happens in his memories, this shows that Cobb is in reality.