When I looked at the American Heart Association web pages^[1], or other, similar pages, I was clearly informed that olde fogies should expect their max heart rate to have declined by about
            220-age.

Yes, everyone is different.  There are ancient fogies, older than 65, who brag that they've had their heart rate up to 170.  Others will brag that their heart rate never, ever gets above 160 and they're only 30.  Yes, everyone is different

***Warning: Determining Maximum rate....****
=> would be very similar to Egyptian Asps -> “very dangerous, you go first”

A person should really have a doctor perform a "treadmill stress test" to determine it. I did that, long ago, the doc was about to stop me when he saw a "glitch" in the ekg of the heart!  He was concerned that it might develop into a full blown heart attack!

How to !!FAIL!!

But I'm found that a sure way to !!FAIL!! is to ignore MHR and the ramifications of it.  I thought I could go nearly as fast now as when I was 20, but it didn't happen.

This was brought home to me when I tried doing group rides in a local bicycle club.  I took the "medium/slower" group, since I was just getting started: 15 mph on the flat.  That about maxed out my speed, for extended time periods.  Seemed like a good fit and I worked but had fun.

The Club held an annual "bike century" and, since I had already been successful in one that summer, I signed up. It was not a huge club and the Century was a bit of a strain, so they set it up that folk would ride in groups: a fast group and a slow group.
=> As a newbie, I took the "slow group, of course, and then I cratered.

What I didn't know was I was the "elder" in the entire group.  The general age in the slow group was in their 20's and 30's and there I am just about twice their age.  They were kind and kept me in the group but they were determined to do the "rated" speed.

So, there I was, go up the hill, my Garmin weeping at me that I was "over max heart rate" trigger, and those turkeys were just cruising and chatting!  I was at 100% of MHR (or, given the number actually "over" unless I was 30's again)!
I was "time trialing" and they were "cruising"... all at the same speed!