How old is Jim from "The Gift of the Magi"?
The answer to your question can be found in the middle of this story, after Della has had her beautiful hair cut off and is waiting for Jim to enter their appartment and is hoping that Jim will not be to angry about her new look now that she has short hair. As Jim enters the appartment, we are given a description of his physical state and presentation:
The door opened and Jim stepped in and closed it. He looked thin and very serious. Poor fellow, he was only twenty-two--and to be burdened with a family! He needed a new overcoat and he was without gloves.
Thus we can see that Jim is twenty-two, and that he gives every appearance of having to struggle to make ends meet, as his frayed and old overcoat and his lack of gloves obviously suggests. In addition, his thin and serious nature likewise indicates the way in which he has to work so hard and try to support both himself and his wife.