How to use this change appeared in Release Notes of Garmin Edge 19.19 version: Enabled manual setting of Resting Heart Rate to enable Fitness Age calculations on Garmin Connect.

I was looking for this too.  I never did find a way to enter it manually but while I was hunting around in the device and app  it I noticed that my resting HR in the Fitness Age page on my device had become set to a value without me entering it.  When I checked, the value is has is exactly what Apple Health has on my iPhone.  This, in my case, comes from my Apple watch. My 540 and Garmin Connect is now showing a fitness age for me and recommending how I can reduce it.    It also needs to know your weight and BMI, it may be that it was me entering those that triggered the Fiteness age 'glance' to update.

Yes, I now have my fitness age too, but it use the RHR I have entered in my HR zones on Garmin Edge (I changed the value to verify)... And Garmin just writes to Apple Health but, in my knowledge, reads no information...