Happy Birthday, Mr. President ... born on Flag Day. Welcome to your 80s. I'm a few years older than you and what lies ahead won't be the most fun you've ever had.
Various bits and bobs around your carcass will begin reminding you of all the years you've enjoyed. Some will hurt, some won't work as well as they did, others will surprise you by soldiering on without a hitch.
Mental acuity can go on undiminished, and what you are doing - using your mind basically non-stop - is probably the best thing you can do to stay sharp.
Stylistically, we couldn't be more different. OTOH, I support nearly all of the many programmatic things you've accomplished.
You've got maybe another 30 months in office. Hang in there and keep punching. We've got your back.