I have cycled all my life - and knew my resting HR was in the fifties as a child and young man. As a teenager I smoked and rock and rolled quite a bit.
In my mid twenties I developed as an adult cyclist and quit smoking and quit the party. By 1980 I started getting serious about training and remember my first "racermate" computer with HR monitor.
My highest known HR was 181 - highest sustained rate (two minutes + was 170 . Resting had dipped to 45-48....
By 1986 my resting HR dropped to 38 and remained there for over 20 years. After I herniated a disk in 2007, I did not workout for months - and the low HR never returned.
At 66yrs by my HR is still in the mid forties naturally, but drops due to beta blocker usage often.
For those cyclists that are dedicated enough to record their AM HR in bed religiously - it can be a useful in determining over training. I always knew I needed some time off when I woke up with an HR 41-42 instead of 38-39.