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.

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.