Minor error in the help section of the fitness age tab.
"...resting heart rate and body fat percentage or BMI"
should be:
"...resting heart rate and body mass index or BMI"
(should this be opened as a support ticket instead?)
"...resting heart rate and body fat percentage or BMI"
should be:
"...resting heart rate and body mass index or BMI"
No, the first one is correct - giving both available options Body Fat % or BMI, while you "corrected" version replaces the Body Fat % with Body Mass Index which the redundant since it is the same as BMI.
With the Fitness Age, you can use both of them. If Body Fat % is available (i.e. you use an Index scale, or import the BF% values manually), it is being used in priority over BMI.