The IRS has released Information Letter 2016-0014 providing guidance on the Maximum Permissible contribution to an HSA in the year an individual reaches age 65.

Document Excerpt

The amount of the Maximum Permissible Hsa contribution in the year an individual attains age 65 is prorated based on the number of months the individual is an eligible individual and whether the individual has self-only or other than self-only coverage. In particular, the maximum permissible contribution is based on the number of months the person in not enrolled in Medicare.

Link

https://www.irs.gov/pub/irs-wd/16-0014.pdf