At the age of 73 my MHR has not dropped. I can still bring it up to 175.
I use a HR monitor on all my rides training, races, and Brevets.
What I have noticed is the HR that I can maintain on a climb has dropped, and keeps dropping every year.
I used to be able to maintain 160-165 bpm on a long climb (45 to 60 min). Now I can only maintain 140-145 bpm on the same extended climb.