What determining factor is crucial for a married couple filing jointly to assess their requirement to file?

When married couples consider filing their tax returns jointly, the primary factor that determines their filing requirement is their combined gross income. The IRS establishes specific income thresholds that, if exceeded by the couple's combined gross income, necessitate filing a tax return. These thresholds can vary based on factors such as filing status and age, and married couples must assess their total income together to determine whether they are required to file.

The other options, while relevant in different contexts, do not serve as the fundamental basis for the requirement to file. For instance, the state of residence may influence state tax obligations but does not affect federal filing requirements directly. The age of the older spouse can impact certain tax benefits or credits, such as the eligibility for certain higher standard deductions or senior tax provisions but does not determine the necessity to file itself. Similarly, the number of dependents claimed may affect the amount of tax owed or refunds available but is not a criterion for assessing whether a couple must file their federal income tax return. Therefore, the combined gross income stands out as the essential determining factor.