Cobb was still in a dream. He told dream Mal that he and she had grown old...spent fifty years down in limbo. We see a shot of two people with obviously aged hands walking hand in hand....yet.....when we see a shot of the train coming towards Mal to force her to wake up from their fifty year sojourn down under, she still looks young. The children? The film didn't shoot for long. How do you make two child actors look "a few months older" when they didn't age that much in real life? That didn't bother me. Had Cobb been away from his children 4 or 5 years you could change the actors, but he wasn't.
Who knows? In reality, Mal, who had jumped off the ledge, might be somewhere desperately willing Cobb to wake up and come back to her in the real world.