Can someone explain the opening scene of Inception where Cobb is shown on the sea shore? He looks weak and is taken by the men of the old guy, who then shares food with Cobb. This old guy looks like Saito, the same guy who hires Cobb to plant the idea in Robert's mind. How did Saito become so old while Cobb looks the same? How can this be possible?
The beginning of Inception is out of order of the actual events of the movie. This scene actually occurs after Saito dies in the third layer of the dream, going into Limbo. Due to the drugs that the dreamers took, time becomes more distorted with each successive layer of the dream, with Limbo having the biggest distortion. A single second in the real world is a life time in Limbo.
What we see at the beginning of the movie is when Cobb first lands in Limbo in order to rescue Saito. Saito is so old because, for him, he's been there for years, even though it's only been a few seconds in the dream layer above.
It is explained towards the end of the movie, when Cobb goes into limbo to save Saito. The concept of dreams-within-dreams is that time dilation between reality and what the person is experiencing inside a dream gets more and more severe with each consecutive layer of dream. The reason it's called limbo is because it's not artificially constructed like the first 3 layers, and so whatever happens in there is very unpredictable, and you are likely to go even deeper since your chances of dying get higher and higher. Limbo is essentially point of no return. Saito is presumably in a VERY deep layer of the limbo, and the time dilation from reality is so stretched out that by the time Cobb went there (probably only a few seconds for Cobb), Saito had grown insanely old.