If you reached age 65 in August, your Medicare Part A coverage began August 1, so with regard to the HSA you are also an ineligible individual for August. You'll need to mark August as being covered by Medicare. This means that you have only 7 months of eligibility and your contribution limit for the year is 7/12 of what you would otherwise be eligible to contribute.
You have until April 15, 2025 to make HSA contributions up to 7/12 of the normal annual limit. Of course if you have already exceeded that, you have an excess that needs to be returned to you by the due date of your 2024 tax return, including extensions, rather than making additional contributions for 2024. (If you are married and your spouse also contributes to an HSA, the amount your spouse contributes to your spouse's HSA can potentially affect the amount that you are eligible to contribute if either of you have family HSA coverage.)
