We can see the scene in Inception where Cobb and Mal commit suicide on the train tracks at a young age, but in the end of the movie Cobb reminds Mal that they lived to a very old age in their dream. How is this possible? Or was the suicide scene just a representation?
Likely the latter: An 'unreliable' representation of what happened. Since that crucial bit of information (that they grew old together) is still left out as Cobb and Mal go over what happened to them in Limbo, this is either Cobb or Mal (or even Ariadne, representing the viewers) imagining they were young there or falsely remembering it. Note that some of the same scenes of them in Limbo (strolling through it holding hands, and their hands on the train tracks) are later reconstructed with their old selves after this bit of dialogue:
Mal: You said you dreamt that we'll grow old together.
Cobb: But we did. We did, you don't remember?
So this is basically a case of a detail left out for the impact of a later reveal; since we don't know it yet, it's not yet visualized - with the possible in-universe excuse that someone is imagining it that way.
The imagery for limbo is a bit confusing, especially when you factor in Sato's appearance when Cobb finally shows up.
That said, however, when they do kill themselves they ARE old, or at least they think they are. I think not making them appear old was a visual inconsistency, but it may have been intentionally made by Nolan to make the whole suicide scene and after more confusing and less clear-cut.