Well it depends. You somehow get to the point of which you just aren't sure what is the real world. Some(although not that many people) may consider that the "they grew old part" was actually the real world, and that Cobb and his wife had this 2 level dream(the first one was where Cobb was with his kids, on the plane, and where his wife "killed" herself).
How can you think that this is what actually happened?
Well you could speculate that someone(it gets just too confusing to talk about if you think who may that someone actually be: Cobb, or his wife) put the idea that "the air plane/children looking back" world is the real one, just so Cobb wouldn't ever leave.
+ If one would to go deeper into speculation to the minor details: we've never actually seen that Cobb's totem was owned in the "real" world by his wife(we don't see Cobb taking it in the children world; we see it only in the make belief one which we know is no real).
To put it bluntly there is no Inception to Cobb's "real" world or "real" totem(which makes you wonder if the last scene is actually a dream or reality).
I guess this just leaves room for interpretation(but I'm just amazed that people are just willing to think and twist their minds to try to speculate on the ending, and that just mostly everyone goes: "well it looked as if it was going to tumble

")