Answer:
The right answer is "While their bodies were young, their minds were old".
The explanation for this answer is that when the author uses the expression "young shoulders" he is referring to a young body, and when he uses the expression "very old heads" he is referring to an old mind.
Probably the author used these expressions to say that although young, the peculiar creatures of Vietnam had a very mature maturity for their ages.
Step-by-step explanation: