It is dramatically striking (at least when its initially done at the VC prison cage), though on repeated viewings when DeNiro goes back to Saigon to find Walken, it becomes more ridiculous than anything else. It’s highly unlikely that a heavily drug-addicted Walken would’ve lasted very long playing that “game” (a day, at the most). Of course, even closer scrutiny and you realize both of them are a decade too old to be playing Vietnam draftees.
The age thing is a long standing Hollywood tradition. One has to imagine they are the right age. Jimmy Stewart was about 20 years too old for the character he plays in Vertigo.