Why can't Joe's son and daughter-in-law claim him as a dependent?

The correct rationale for why Joe's son and daughter-in-law cannot claim him as a dependent lies in the income criteria established by the IRS for qualifying relatives. To be claimed as a dependent, a person categorized as a qualifying relative must have a gross income that does not exceed a certain threshold, which changes yearly. In this case, if Joe's income surpasses the stipulated limit, it disqualifies him from being claimed as a dependent by his son and daughter-in-law.

While factors such as Joe's age, living situation, or retirement status are relevant in discussing dependency claims, they don't address the specific requirement related to income that is critical in this scenario. Therefore, the verification of Joe's income is the key element that prevents his son and daughter-in-law from claiming him as a dependent.