My understanding is that heart rate is very variable (within a person), higher could mean you are 'fresh' or have a virus or another issue), lower than normal could mean that you are fitter - (larger stroke volume), or you could be fatigued, and can't seem to get your heart rate up like 'normal'. I would also take google with a pinch of salt. We are all different. And while we do lose max hr over decades, your wife's hr seems well within range of a race winning 51 yo athlete. Maintaining a high max hr as you age is, to my knowledge, not necessarily a bad thing. My max 20 years ago, at 44, was 187, and my highest ahr for a 2 hour race then was 173. Now, at 64 my max is between 175 and 181 depending on how trained or rested I am. This week I could only manage 173 in a 5 min max effort, but this after a 743 tss week, and my power was within 5% of my best for 5 years. 3 months ago doing a similar test I got to 180 but a lower power number. I was not as fit then as I am now.
I do not believe that pushing yourself as you age is necessarily a bad thing. NB atrial fib, though, is another thing to be aware of, particularly as you age.
