I just seemed to do the same thing recently. I'm 67. Between last year and this year I seem to be down 10 bpm on my max. Partly I put it down to some extended times I was not able to ride last year that normally I would have been riding. And I never got back up to my peak performance last year either.

I'm not worried about it. It just is what it is. If at your age you aren't seeing a cardiologist somewhat regularly, then I'd suggest you do.