The phrase in the excerpt from Elizabeth Bishop's "The Fish" which compares the fish to an old, experienced soldier, is the following one:
"Like medals with their ribbons frayed and wavering, a five-haired beard of wisdom trailing from his aching jaw."
The author mentions the medals, which are a reference to soldiers and their deeds of glory. Plus, the ribbons of the medals are worn out, surely a sign of time and experience. There is also the metaphor "beard of wisdom", which is a way to compare the fish to an old and wise individual. Not to mention the "aching jaw", certainly a reference to soldiers who suffer from permanent damage as a result of the battles they have fought.