So there are some wrinkles here.
1. You should not decide based just on taxes. How else can your spouse and child get insurance coverage? Is the HDHP the best option?
If you are working at age 70, and you enroll in a family HDHP, then your wife can contribute to an HSA because she is "covered" by a family plan, even if she is not the "owner" of the family plan. As long as she is not covered by any other type of insurance.
2. Once you own your own HSA, you can use the funds for qualifying medical expenses for yourself, your spouse, and your dependents, no matter what kind of coverage you have. Ability to spend is not connected to eligibility to contribute. If you are covered by Medicare, you CAN reimburse yourself for Medicare premiums you pay (if you choose), but not Medigap insurance. You can also pay for out of pocket expenses for yourself, your spouse and your dependents from your HSA. You can't reimburse yourself for your workplace premiums since they are almost certainly already deducted from your pay pre-tax.
3. If your spouse is "covered" by a family HSA, and has no other medical coverage, her contribution limit for 2026 is the family limit of $8750, plus $1000 catch-up if she is age 55 or older.
4. You or your spouse can use funds from your HSAs to pay for medical expenses for self, spouse, or any dependents. This means tax dependents. A child who is a tax dependent can not contribute to their own HSA, but you can cover their expenses. (Remember, an HSA is owned by one person only.)
Now here's the wrinkle. Because you can cover a child on your medical policy up to age 26, but a child can't be a tax dependent if they are 24 or older (or if they are 19 and older but not a full time student) there is a "window" where the child can be covered by your insurance but is not a tax dependent. In that window, you and your spouse can't pay for their medical expenses from your HSAs, but the child can contribute in an HSA in their own name and then use the funds to reimburse themselves for medical expenses. Their contribution limit would also be $8750 if they are covered by a family plan, and their limit is not reduced by any contributions you or your spouse make. It's a kind of loophole caused by the fact that the age to be a dependent stops at 24 but the age to be covered by parent's insurance is age 26.
