65 years old and retired. Lost 3 pounds and the heart rate dropped 10. Was always in the mid 60's since in my 20's, now in the mid 50's. My friend who is a silver olympic medalist in the 68 games remarked to me a few months ago that she can no longer get her heart rate up past 120. She is 73 and has been focused on athletic performance since age 13.
I think it is likely you are aging just fine.