Age and experience dictates much of the behavior of both the young fishermen in this story and the old fishermen.
The young fishermen are making fun of Santiago because they see his 84-day streak without a fish as proof that he is past his prime and no longer any good at what he does. They are too young and inexperienced to realize he may just be having a dry spell, but that does not make him a bad fisherman or in need of retiring.
For his part, Santiago does not get mad at them because he has the wisdom to understand where they are coming from, and he also knows better than to agree with them. He is far more experienced than they are and he probably knows all too well that fishing can be a feast or famine endeavor, meaning that sometimes you have more than enough and other times you nearly starve. Because of this, a dry spell to the younger fishermen might mean something different than to Santiago and the older fishermen, who understand that a dry spell will pass.