Here, again, Paul is trying to capture the sheer number of conflicting emotions that plague him day and night. If you break it down into its components, you can make sense of the whole:
"Forlorn like children," = pitiful, sad, abandoned, lonely, and very much without a sense of direction (either in the war or in life).
"experienced like old men," = these soldiers experienced more death in one day than most experience in a lifetime. Add to this the kind of raw instinctual wisdom it must have taken to survive, and in every way, though young, these boys feel very old.
"crude and sorrowful and superficial" = certainly this one is more ambiguous, but contextually, consider that each resting day together is a gift. They laugh and joke and play games, their humor is likely coarse or boyish "toilet humor" and because it is one of the only emotional means of escape (and infrequent), it is often crude. But then, it is laced with sorrow because it is the reminder that normal still exists, somewhere, for someone. The thought of another battle reminds them that this moment of normalcy is short lived, always. Further, the death of many who may have been joking only days before is another moment of sorrow, as well as the realization that this might be the last time to joke. Finally, superficial, because, in many ways, the light hearted times, wedged between battles at the front (and more death), are only passing moments that bring them back to death again.
"lost" = given the mixture of extremes, Paul's final assessment of the depth of his experience and emotion can only be summed up in this word.