On the first day of class, we see a middle-aged man at the front of the room, talking to a younger man. if we assume the older man is the professor and the younger man is the student, we are relying on what heuristic?

If we assume the older man is the professor and the younger man is the student, we are relying on   the representativeness heuristic
Representativeness heuristic refers to a judgment that we made based on the probability under an uncertain event.
In this case, since the man is older, we can safely assume that that person has more academic experience than the younger one.