If you are turning 65, you are likely going to be enrolling in Medicare. Once you are enrolled in Medicare you may no longer contribute to your HSA. You lose eligibility to do so as of the first day of the month you turn 65. So if you turn 65 on June 21, you may not contribute to your HSA after June 1.
Your maximum contribution for the year will be 5/12 (for the five months of January through May) times the contribution limit of $4,450 ($3,450 plus a $1,000 catch-up contribution allowed for those over age 55).