An individual is subject to the family limit if that individual is an eligible individual and the plan that covers that individual also covers at least one other individual, whether or not that other individual is eligible to contribute to an HSA. However, it's often the case that the a plan that originally covered both spouses switches to self-only coverage when one of the spouses becomes covered by Medicare and the self-only limit would apply to that month and beyond, but an automatic switch to a self-only plan upon one spouse becoming covered by Medicare depends on the terms of the particular plan. Section 223(c)(4) of the tax code defines "family coverage" as any coverage other than self-only coverage.
