If we could see your stroke volume that would give a better picture of any decline. Perhaps you heart is stronger and larger and has adapted to pump a larger stroke per beat, making high HR relative? I'd guess you are still physiologically stronger than guys half your age.

Probably something to all that, this morning my resting heart rate was 36 BPM. That's not so unusual for an endurance athlete but sometimes the nurses at the doctor's office are surprised by that low a number.