Outside the window, it began to rain,

The original plan is to have the floor of the hotel room drop out from underneath the dreamers and thus provide the 'kick' that wakes them up - specifically Eames who is 'hosting' the dream in the snowy mountains. This is supposed to happen before the van plows through the barrier and off the bridge. However, the timing is thrown off and now that Arthur's body is in free-fall in the van, it translates into his dream of the hotel - thus preventing the floor from dropping once the explosives discharge. He improvises a kick by moving everyone into the lift and then setting off an explosive to cause the lift to start moving up the shaft (this works much like a rocket on a space craft). To those in the lift, the force of the lift pushing them is equivalent to gravity. Gravity alone is not enough to awaken the dreamers, but when the lift hits the roof at the top of the shaft, the sudden stopping or slowing-down causes the dreamers to first be propelled upwards, and then fall back to the floor. The result is the same as the inner ear only detects the acceleration due to a force acting on the body, much like Eames' demonstration of tipping Arthur's chair over to wake him up.

Chronologically, Saito entered limbo before Cobb did. So it makes sense that Saito should have aged more than Cobb.. another theory is that Cobb entered limbo twice. The first time was with Ariadne. They found Fischer and kicked him back to Eames' dream. Meanwhile Saito dies and goes to limbo. Cobb provokes Mal into attacking him. (NOTE - he provoked her very methodically. This was a conscious choice he made for some reason and this theory is one explanation for it.) Mal dies from Ariadne's gunshot wound and Cobb dies from Mal's knife wound. At this point, Cobb is currently being dying in all 3 levels (in 3: crushed by the exploding snow fortress, in 2: in the crashing elevator, in 1: left to drown in the van.) When he dies in limbo, he wakes up in one of (all of?) these levels and dies again, sending him to limbo and washing up on the shore near Saito's palace. Saito, having been in limbo this whole time, has aged decades while Cobb is just arriving to limbo for the 2nd time. A second perspective as to why Saito is older, is that limbo is formed around the "raw subconscious." During the helicopter scene, Saito relates to Cobb his personal fear of becoming an old man filled with regret. And this is the form that Saito takes in his limbo. His age does not reflect the time in the various levels of dreams. With this in mind, the flashbacks of Cobb and Mal in their old age perhaps did not reflect them becoming old in limbo, rather it was real memories of themselves.