Everything we were shown in the ending WAS the real world and not just a dream to help Cobb or keep him stuck in the subconscious or some other thing. Him meeting his kids WAS real. The top wobbles because it IS a real top, as a wobbling top can’t stay wobbling forever. HOWEVER, the cut-to-black is a metaphor. Have you ever noticed that almost every time our dreams are about to end or reach a fulfilling point, we wake up before it does? That’s Nolan cutting to black. Nothing else. Just a metaphor. It just a happy ending with a cute little wink from the director giving us a wake-up KICK.
Saito… My interpretation: You can’t get old in a dream unless you believe you are old or if you WANT to be old. We know he doesn’t want to be an “old man, filled with regret,” so that’s out. It probably just took a long time for Cobb to get there. “Why isn’t he as old as Saito?” Because Saito felt the unbearably slow passage of time, not knowing whether he’d be rescued or how long it has been in the real world. In a dream, age is perception. Cobb, on the other hand, IS ACTUALLY SLIGHTLY GRAY. I almost missed it myself, but trust me, it’s there. Mal and Cobb are not gray on the train tracks, though. They should be 80-something by then, but because they both remember that it’s not real, they don’t look as old as they do from behind as they walked (remember that scene?) Cobb is only slightly grey because he perceives a long time of wandering, and he perceives himself a little less youthful, but not much.
Incidentally, we don’t know how old Saito is, especially since the Japanese have the longest lifespans on the planet. I sincerely don’t think it’s coincidence that Nolan chose a Japanese person, as it is convenient because it allows some liberty in the amount of time it took for Cobb to reach him. Think of it: if you are having one of those dreams where you can control stuff (lucid dream) you can’t just snap your fingers and go to Japan, nor can you make an infinite passageway like Ariadne did unless you use the logic of mirrors. You’d have to use logical reasoning to make a 10000 mile long bridge, or swim all the way to Japan (which almost seems like he did), or build a plane from scratch. Even THEN, Cobb had to figure out where he was in Japan in the first place. All this took time, and it took a tiny toll on him compared to Saito, who could do nothing but wait.
The Kick… I think Cobb was bending the truth about how hard it is to get back from Limbo. I think he just said that because he didn’t want HIMSELF to go to Limbo, knowing he’d be the only one tempted to stay there. He knew full well that he would run into Mal, so he wanted to avoid leading the team there at all costs. Think about it: all you have to do to get out is be hit by a train???? The team could have escaped Limbo in seconds. But not COBB. ^_^ Once he was free from Mal (only with the unforeseen help from Ariadne), he knew that he and Saito were just two bullets away from waking up.